Chapter 24 Microbes in Human Welfare (Microbiology) by TEACHING CARE online tuition and coaching classes

Chapter 24 Microbes in Human Welfare (Microbiology) by TEACHING CARE online tuition and coaching classes

File name : Chapter-24-Microbes-in-Human-Welfare-Microbiology.pdf




Microbiology is the branch of science, which deals with the study of microorganism and their process is called as microbiology. Antony Von Leeuwenhoek is known as father of microbiology and father of modern microbiology is Robert Koch. Microbiology is the study of living organism of microscopic, which include bacteria, fungi, algae, protista, viruses, etc. It is concern with their forms, structure, reproduction, physiology, metabolism and classification. It includes the study of their distribution in nature and relationship to other living organism. Their effects on human beings and on other animals and plants. Their abilities to makes physical and chemical change in our environment.


Study of bacteria is called bacteriology. Linnaeous placed them under genus vermes. Nageli classified bacteria under schizomycetes. Bacteria are unicellular, microscopic organisms. These are the smallest cellwall having prokaryotic cell. They differ from animals in having a rigid cell wall and being capable to synthesize vitamins. Bacteria were first seen by a Dutch lens maker, Antony Von Leeuwenhoek (1683) who named them animalcules. Louis Pasteur (1822-95) made a detailed study of bacteria and proposed germ theory of disease. Ehrenberg (1829) was the first to use the term bacterium. Robert Koch (1881) found that some diseases like tuberculosis, cholera in man, and anthrax in cattle is caused by bacteria. Lister introduced antiseptic surgery he used carbolic acid for sterilization of surgical instrument. Pasturization theory was proposed by Louis Pasteur.

  • Occurrence and Distribution : The bacteria constitute a highly specialised group of one celled They are cosmopoliton. They flourish in our mouth and intestine. They live in the bodies of other organisms and their dead remains. Bacteria are not found in healthy blood, depth of some feets in the soil fire and healthy cell. Some thermophilic bacteria can tolerate the temperature upto 78oC while in psychrophilic bacteria occurs upto the temperature of – 190oC. The features which contribute to their universal distribution are –
  • Extremely simple
  • Small size and consequent large surface–to–volume ratio. In order to maintain their small size, cell division occurs
  • Resistance of vegetative cells to adverse environmental Such as U.V. light desication. etc.
  • Formation of highly resistant
  • Diversity in their modes of
  • Plant characteristics : The bacteria are microorganisms that possess rigid cell wall and when motile have They are unicellular organisms lacking true nucleus and membrane bounded cell organelles. The plant characteristics of bacteria are –
    • Presence of a definite and rigid cell wall which in a few species contains
    • The tendency of some to grow as
    • The ability of autotrophic bacteria to synthesize organic food from inorganic materials such as CO2 and
    • Structure of the bacterium cell and reproductive methods are similar to that of certain
    • Ability to synthesize amino acids from inorganic





  • Size : Bacteria are the smallest of all known cellular organisms which are visible only with the aid of They are 3 to 5 microns (1 m = 1/1000 millimetre or about 1/25,000 inch) in length. A few species of bacteria are approximately 15m in diameter.
  • Shape : The shape bacteria usually remain constant. However, some of them are able to change their shape and size with


changes in environmental conditions. Such bacteria, which change their shape, are called pleomorphic. The bacteria possess the following forms.

  • Cocci : (GK. Kokkos = Berry) They are oval or spherical in They are


Tetracoccus Diplococcus


Streptococcus Coccus bacteria

Staphylococcus    Sarcina


called micrococcus when occur singly as in Micrococcus, diplococcus when found in pairs as in Diplococcus pneumoniae, tetracoccus in fours, streptococcus when found in chains as in Streptococcus lactis, staphylococcus when occurring in grape like clusters as in Staphylococcus aureus and sarcine, when found in cubical packets of 8

Bacillus                 Diplobacillus   Palisade Bacillus


Bacillus bacteria


















Stalked          Budded


or 64 as in Sarcina.

Fig Different forms of bacteria


  • Bacilli : They are rod–shaped bacteria with or without They may occur singly (bacillus), in pairs (diplobacillus) or in chain (streptobacillus).
  • Vibrios : These are small and ‘comma like, kidney They have a flagellum at one end and are motile, vibrio bacteria has curve in its cell e.g., Vibrio cholerae.
  • Spirillum (Spira = Coil) : The spirillum bacteria (plural-spirilla). They are spiral or coiled like a cork- The spirillar forms are usually rigid and bear two or more flagella at one or both the ends e.g. spirillum, spirochaete, etc.
  • Filament : The body of bacterium is filamentous like a fungal The filaments are very small e.g.

Beggiota, Thiothrix etc.

  • Stalked : The body of bacterium posses a stalk g. Caulobacter.
  • Budded : The body of bacterium is swollen at places g. Retrodomicrobiom.
  • Flagellation : Depending upon the

presence or absence of flagella, the bacteria are of following types :–


  • Atrichous : When the flagellum is absent it is called e.g. Pasturella,

A                B                C                        D                           E

Fig : Different types of bacteria on the basis of flagellation : (A) Atrichous

  • Monotrichous (C) Lophotrichous (D) Amphitrichous (E) Peritrichous






  • Monotrichous : Only one flagellum is found at one e.g. Vibrio, Cholerae.
  • Lophotrichous : When a group of flagella is present at one end g. Vibrio.
  • Amphitrichous : When single or group of flagella is present at both the end g. Nitrosomonas.
  • Peritrichous : A number of flagella are present all over the e.g. E. coli.

(6)  Staining of bacteria

  • Simple staining : The coloration of bacteria by applying a single solution of stain to a fixed smear is termed simple staining. The fixed smear is flooded with a dye solution for a specified period of time, after which this solution is washed off with water and the slide blotted The cells usually stain uniformly. However, with some organisms, particularly when methylene blue is used, some granules in the interior of the cell may appear more deeply stained than the rest of the cell, indicating a different type of chemical substance.
  • Gram staining : This technique was introduced by Hans Christian Gram in It is a specific technique which is used to classify bacteria into two groups Gram +ve and Gram –ve. The bacteria are stained with weakly alkaline solution of crystal violet. The stained slide of bacteria is then treated with 0.5 percent iodine solution. This is followed by washing with water or acetone or 95% ethyl alcohol. The bacteria which retain the purple stain are called as Gram +ve. Those which become decolourised are called as Gram –ve. In general the wall of Gram +ve bacteria have simpler nature as compared to


Gram –ve bacteria. E.coli is a Gram –ve bacterium. Gram negative bacterium can be seen with other stain safranin.

The most plausible explanations for this phenomenon are associated with the structure and composition of the cell wall. Differences in the thickness of cell walls between these two groups may be important the cell walls of Gram-negative bacteria are generally thinner than those of Gram- positive

bacteria. Gram-negative bacteria contain a higher percentage











Plasma Membrane





Mesosome   Peptidoglycan



Teichoic acid + lipoteichoic acid



of lipid (11 to 22%) than do Gram-positive (1 to 4%), bacteria, Experimental evidence suggests that during staining

Fig : Difference between cell walls of Gram-negative and Gram- positive bacteria


of Gram-negative bacteria. The alcohol treatment extracts the lipid, which results in increased porosity or permeability of the cell wall. Thus crystal violet- iodine (CV-I) complex can be extracted and the color of the safranin counterstain. The cell walls of Gram-positive bacteria, because of their different composition, lower lipid content, become dehydrate during treatment with alcohol. The pore size decreases, then permeability is reduced, and the CV-I complex cannot be extracted. Therefore these cells remain purple-violet.



S.No.Gram – PositiveGram – Negative
(1)Cell wall thick (250 – 300 Å).Cell wall thin (100 – 150 Å)
(2)Cell wall homogenous.Cell wall heterogenous.
(3)Cell wall single layered.Cell wall 3-layered.
(4)Cell wall more rigid.Cell wall less rigid





(5)Cell wall made up of mucopeptide (80%).Cell wall made up of lipoprotein, lipopolysaccharide and mucopeptide.
(6)Teichoic acid (5 – 10%) present.Teichoic acid absent.
(7)Spore producing forms included.No spore producing form.
(8)Polar flagellum usually absent.Polar flagellum usually present.
(9)Contain Mg-ribonucleate.Mg-ribonucleate absent.
(10)Not soluble in 1% KOH.Soluble in 1% KOH.
(11)May produce exotoxins.May produce endotoxins.
(12)Sensitive to penicillin.Not sensitive to penicillin.
(13)L-lysin present in peptideDiamino palmilic acid present in peptide.
(14)O-antigen absent.O-antigen present.


(7)  Structure of bacterial cell

  • Capsule : In a large number of bacteria, a slimy capsule is present outside the cell It is composed of polysaccharides and the nitrogenous substances (amino acids) are also present in addition. This slime layer becomes thick, called capsule. The bacteria, which form a capsule, are called capsulated or virulent bacteria. The capsule is usually found in parasitic forms e.g. Bacillus anthracis, Diplococcus pneumoniae, Mycobacterium tuberculosis.

Function of capsule

  • It provides protection against phagocytosis and
  • Capsule also protects the cell against dessication and viral

Type of capsule

  • Homopolysaccharide : When capsule are made by one type sugar g. Streptococcus mutans.
  • Heteropolysaccharide : When capsule are made by many type sugar g. Streptococcus pneumonae.
    • Cell wall : All bacterial cells are covered by a strong, rigid cell Therefore, they are classified under plants. Inner to the capsule cell wall is present. It is made up of polysaccharides, proteins and lipids.
  • In the cell wall of bacteria there are two important sugar derivatives are found e. NAG and NAM (N-acetyl


glucosamine and N-acetyl muramic acid) and besides a- or D – alanine, glutamic acid and diaminopimelic acid are also found.

  • One of the unique components of cell wall of bacteria is peptidoglycan or mucopeptide or murien (made of mucopolysaccharide + poly peptide).










Cell wall

Plasma membrane


chain      Cytoplasm








  • In peptidoglycan, NAG and NAM are joined by short peptide chains or cross bridges of amino



Free enzyme




Nucleoid DNA
  • Outer layer of cell wall of Gram –ve bacteria is

Fig : Electron microscope structure of a bacterium cell




made up of lipopolysaccharides and cell wall of Gram +ve bacteria of teichoic acid.

  • The cell wall of Gram positive bacteria is much thicker and contains less lipids as compared to that of Gram +ve
    • Plasma Membrane : Each bacterial cell has plasma membrane situated just internal to the cell It is a thin, elastic and differentially or selectively permeable membrane that allows passage of dissolved substances in and out of the cell. It is composed of large amounts of phospholipids, proteins and some amounts of polysaccharides but lacks sterols. The plasma membrane of bacteria provides site for most of the anabolic and catabolic pathways. It is characterised by possessing respiratory enzymes, which are bound to its inner surface some exoenzymes are also associated with its outer surface which catalyze digestion of insoluble materials.

(a) Mesosome : On the plasma membrane generally at mid point, there are present some circular coiled bodies called mesosomes. If plasma membrane is stretched then mesosomes are disappeared. So mesosomes are simply infoldings of plasma membrane. Mesosomes contain respiratory enzymes like oxidases and dehydrogenases and hence they help in respiration. Hence mesosomes are also known as “mitochondria of bacterial cell” or chondrioides. Mesosomes are more prominent in Gram +ve bacteria.

  • Mesosomes are present at mid point, so they help in equal distribution of nuclear material during binary
  • It help in secretion and synthesis of material for cell
  • It receive DNA during conjugation and DNA replication
  • Mesosome participate in the formation of septa during cell
  • Cytoplasm and cytoplasmic inclusions : The cytoplasm is a complex aqueous fluid or semifluid ground substance (matrix) consisting of carbohydrates, soluble proteins, enzymes, co-enzymes, vitamins, lipids, mineral salts and nucleic The organic matter is in the colloidal state.

The cytoplasm is granular due to presence of a large number of ribosomes (about 20,000 to 30,000), which occur singly or in small groups called polyribosomes. The ribosomes in polyribosomes are held together by means of messenger RNA. The ribosomes of bacteria are smaller (70S) as compared to those of eukaryotic cells. Ribosomes in bacteria are found in the form of polyribosome. Membranous organelles such as mitochondria, endoplasmic reticulum; Golgi bodies, lysosomes and vacuoles are absent. In some photosynthetic bacteria the plasma membrane gives rise to large vesicular thylakoids which are rich in bacteriochlorophylls and proteins.

  • Volutin granules : These are so called because they were first reported in Spirillum volutans bacteria. These are also known as metachromatic granules, which are composed of They stain an reddish purple colour with dilute methylene blue. By electron microscopy they appear as round dark areas. Volutin serves as a reserve source of phosphate.
  • Fatty acids granules or poly-b-hydroxy butyric acid granules (PHB) : These are polymer of lipid like material and chloroform soluble which are often found in aerobic bacteria especially under high carbon low nitrogen culture conditions. Granules can serve as a reserve carbon and energy source. PHB granules can be stained with lipid soluble dyes such as nile By electron microscopy they appear as clear round areas.
  • Glycogen and sulphur granules : Glycogen are also known as polysaccharide granules. It can be stained brown with By electron microscopy they appear as dark granules. Another type of inclusion is





represented by the intracellular globules of elemental sulfur that may accumulate in certain bacteria growing environments rich in hydrogen sulfide.

  • Nucleoid : It is also known as genophore, nacked nucleus, incipient nucleus. In contrast to eukaryotic cells, bacterial cells contain neither a distinct membrane enclosed nucleus nor a mitotic apparatus, However, they contain an area near the center of the cell that is regarded as a nuclear structure. There is present nuclear material DNA in bacteria is double helical and circular. It is surrounded by some typical protein (polyamine) but not histone proteins. Histones (basic proteins) are altogether absent in bacteria. This incipient nucleus or primitive nucleus is named as nucleoid or genophore.
  • Plasmid : In addition to the normal DNA chromosomes many bacteria (g. E.coli) have extra chromosomal genetic elements or DNA. These elements are called plasmids. Plasmids are small circular double stranded DNA molecules. The plasmid DNA replicates independently maintains independent identity and may carry some important genes. Plasmid terms was given by Lederberg (1952). Some plasmids are integrating into the bacterial DNA chromosome called episomes. Plasmids are following type.
  • F-factor or fertility factor or F-plasmid : Which is responsible for transfer of genetic material from donar to recepient
  • R-factor or resistance factor or R-plasmid : It provides resistance against drugs. Some of the R- plasmid can be transferred to other cells by conjugation, hence the term infectious resistance. Each form of resistance is due to a gene whose product is an enzyme that destroys a specific
  • Colicinogenic factor : Which produces ‘colicines’ which kill other bacteria (other than which produce these colicines).
  • Flagella : These are fine, thread-like, protoplasmic appendages which extend through the cell wall and the slime layer of the flagellated bacterial cells. These help in bacteria to swim about in the liquid medium. Myxobacteria donot has flagella and move by gliding movement. Bacterial flagella are the most primitive of all motile Each is composed of a single thin fibril as against the 9+2 fibrillar structure of eukaryotic cells. It consists of a few fine fibrils twisted tightly together into a rope-like helical structure. The flagellum is composed entirely of flagellin protein.


According to Low and Hanson (1965), bacterial flagellum is composed of globular subunits arranged in helices of various kinds.

The diameter of each subunit is about 40-50Å. These subunits are arranged around a hollow axis. A flagellum is usually 4.5 m long and 120-185 Å in diameter. Flagellum is attached to cell membrane by a special terminal hook, which is attached to the basal body called (bleferoplast). A bacterial flagellum can be divided into three parts.

  • Basal granule : It is like a rod it lies with in the cell wall and cell membrane and bears ring like swellings in these










L ring




M ring

















Peptidoglycan loyer

Outer membrane Peptidoglycan layer

Cytoplasmic membrane


  • A hook : It represent the middle and thickest part of Fig : Structure of flagella





flagellum. Hook is curved tubular structure which connects the filament with the basal body.


  • Filament : It represents cylindrical hollow structure made up of protein


  • Pili or Fimbriae : Besides flagella, some tiny or small hair-like outgrowths are present on bacterial cell These are called pili and are made up of pillin protein. They measure about 0.5–2mm in length and 3–5mm in diameter. Pilin are arranged helically around a central hollow core. These are present in almost all Gram-ve bacteria and few Gram +ve bacteria. These are of 8 types I, II, III, IV, V, VI ,VII, and F types. I to F are called sex pili.



  • The function of pili is not in motility but they help in the attachment of the bacterial


  • Some sex pili acts as conjugation canals through which DNA of one cell passes into the other


  • Normal Growth cycle or Growth curve of bacteria : When we inoculate a fresh medium with a given number of cells, determine the bacterial population intermittently

during an incubation period of 24h (more or less), and plot the logarithms of the number of cells versus time, we obtain a curve of the type illustrated figure from this it can be seen that there is an initial period of what appears to be no growth (the lag phase), Followed by rapid growth (the exponential or logarithmic phase), then a leveling off (stationary phase), and finally a decline in the viable population (death or decline phase). Between each of these

phases there is a transitional period (Curved portion). This


represents the time required before all cells enter the new phase.




  • Reproduction in bacteria : Methods of reproduction are

Vegetative reproduction

  • Budding : It is a rare method of reproduction and is reported in Bigidi bacterium bifidus.


Fig : Growth curve of bacteria






Cell wall                                 Cell membrane


DNA                        Cytoplasm


Parent cell




E                                                         B


  • Binary fission : It is the most common type of reproduction in bacteria during favourable When the conditions of food, water and temperature are favourable. Here bacterial cell divides by a constriction into two halves. At the same time nuclear material elongates and divides into 2 equal

New formed two parts separate apart from each other and give rise to two new cells





New formed two                D

parts start to

separate apart

Bacterial cell expands







C               Due to the formation of a transverse septum, cytoplasm

divides into two parts


Fig : Different stages in the binary fission of a rod shaped bacteria





halves probably helped by mesosomes. During this process, the single circular chromosomes duplicates it self a long with DNA duplication under favourable conditions of binary fission. Bacterial cell divides into two after every 20 minutes and at this rate in 24 hours period, a single bacterial cell produces 4×1021 bacteria, but only about 10% of them survive, The speed of binnary fission is decreased due to low temperature. Therefore, food is preserved in the cold storage. The cause of food spoilage and bacterial infection is the rapid multiplication of bacteria.

Asexual reproduction

  • By endospore formation : During unfavourable condition, highly resistant single spore is formed inside the bacterial cell, which is known as (Endo


means inside or within + spore) Endospore means spore inside bacterial cell or cell inside cell.

  • Endospore formation is more common in rod- shaped bacterial or bacillus Position of single endospore may be terminal or sub-terminal or intercalary.
  • Endospore is having a characteristic structure, e., having outer thin exosporium followed by one or many layered spore coat, followed by cell many concentric layers










Bacterial cell wall










Bacterial plasma membrane










Bacterial cytoplasm


Spore coat


Cell wall or core wall

Plasma membrane Spore cytoplasm


of cortex, which if followed by cell wall, cell membrane and matrix.

Fig : Detailed structure of endospore


  • Endospore is highly resistant to very high and very low temperature, strong chemicals and acids, etc., due to calcium dipicolinic acid and peptidoglycan in Dipicolinic acid (DPA) helps in stabilizing its proteins. DPA and Ca ions provide resistance to heat.
  • When favourable conditions come, outer layers rupture and active bacterial cell comes So this is a method of perennation (i.e., to tide over unfavourable condition) and some people say it “ reproduction wihtout multiplication”.
  • The bacterial spore or endospore is perhaps the most resistant living structure known to
  • Tetanus causing and anthrax causing bacteria produce
  • By conidia : These are found in filamentous bacteria like streptomyces. The conidia are spore like structure formed in Each conidium gives rise to a new bacterium.
  • By zoospores : Motile spores are formed in Rhizobium bacteria, but are rare in other bacteria

Sexual reproduction (Genetic recombination or parasexuality)

Sexual reproduction in bacteria is not of the kind as found in eukaryotic organisms. In case of bacteria, the sex organs are not formed, meiosis and mitosis does not occur, the two gametes do not fuse with each other and the diploid zygote (having two set of chromosomes within a true nucleus) is not formed. Instead, a portion of genetic material (DNA) is transferred from a ‘donor’ cell (male) to a ‘recipient’ cell (female) making it an; incompletely diploid zygote. The process is actually called genetic ‘recombination’ which occurs in three ways.

  • Transformation : In this process one kind of bacterium is transformed into another kind. It takes place by transferring DNA from one bacterium to another It was first reported by Griffiths (1928). Avery, Mcleod and Mc Carthy (1944) perform a detailed study of transformation in Diplococcus pneumoni. In this experiment one





type bacteria are virulent (pathogenic) having an extracovering of polysaccharids. These are called capsulated bacteria or (rough bacteria) or R-bacteria. Another type are avirulent non-pathogenic are called non-capsulated bacteria or (smooth bacteria) or S bacteria. This experiment was completed in 4 steps.


  • Avirulent strain
  • Virulent strain
  • I¾nje¾ctin¾m¾ice® Healthy mice.
  • I¾nje¾ctin¾m¾ice® Mice di


  • (Heat killed virulent strain) Bacterial strain ¾¾Inje¾ct in¾m¾ice® Healthy mic
  • Avirulent +(Heat killed virulent bacterial strain) ¾¾Inje¾ct in¾m¾ice® Mice die

In his experiments, Griffith mixed R-types with the heat killed S-type cells and injected them into a laboratory mice. He observed that non- capsulated R-type cells became converted into capsulated types. This shows that a small portions of DNA from heat killed S-type cells have entered into non- capsulated R-type cells and transformed


them into capsulated types.






Heat killing

Non Capsulated









Fig    :    Transformation    of    a    non    capsulated

pneumococcus bacterium into a capsulated type

Transformation are not common in nature because the large fragments of DNA molecules can not pass through the recipient’s cell walls or membranes, However, this process has been made possible experimentally by


protoplast fusion and other related techniques. It has been shown that small amount of DNA (i.e., less than 5% of the total genome) is actually transferred during transformation. Some of the important

Phage DNA



Bacterial cell 1


characters transferred from one bacterial cell to another bacterial cell by transformations are development of pathogenicity, drug resistance, formation of capsules and change in the nutritional patterns.

  • Transduction : Transduction is the process in which the genetic material (a portion of DNA) of one bacterium is transferred to another through the agency of temperate (lysogenic) bacteriophage (e., bacterial virus). The process was discovered by Zinder and Lederberg (1952) in bacteria-Salmonella typhimurium.

During this process a donor bacterial cell gets infected with a bacterial virus. The viral DNA, instead of multiplying itself, becomes associated and integrated with bacterial DNA. Thus the genes of

bacterium get linked with the genes of virus. It is followed by the





Bacterial DNA




Bacterial DNA




Bacterial DNA



Phage DNA


Bacterial cell 2 Transducted



multiplication of virus inside the bacterial cell.

Fig : Transduction where fragment of one bacterial cell is passed on to another bacterial cell through the agency of a phage





Bacterial cell resulting in the formation of normal (containing viral DNA) and defective (containing broken fragments of the host DNA) bacteriophage. The defective viruses containing the fragments of bacterial DNA are liberated along with the normal viruses after the lysis of bacterial cell. These viruses attacks the other bacterial cells infection of a recipient cell by a normal bacteriophage usually leads to lysis. A few recipient bacterial cells, however, become infected with a defective transducing bacteriophages. Thus the viral DNA, which consists of bacterial DNA, gets associated and integrated with the recipient bacterial cell completing the process of transduction. In this way, the DNA fragment of one bacterial cell is transferred to another bacterial cell.

Transduction has been observed in many bacterial genera such as Salmonella, Escherichia, Shigella, Bacillus, pseudomonas, etc. Two kinds of transduction can be distinguished.

  • Generalised (non-specific) transduction which can transfer any phage sized fragment of host
  • Specific transduction which is restricted to the transfer of specific portion of
  • Conjugation : Transfer of DNA by the process of conjugation was first described by two American scientists Lederberg and Tatum (1946) in Escherichia coli. It occurs between two sexually different strains of the bacteria (coli)- one acts as donor of genes (male) and the other as recipient of genes (female) both are haploid. The donor (or male) cells prossess sex-factor or fertility factor (F-factor). F-factor is a small genetic particle of circular DNA. It replicates at the time of cell division and inherited by the progeny. The F-factor codes for the special type of protein that determines the formation of sex pili in donor cells and formation of conjugation bridge or conjugation tube between the donor and recipient cells. The F-factor may remain free in the cytoplasm (i.e., independent of bacterial chromosome) or it may be integrated with the bacterial chromosome. If it remains free in the cytoplasm, the bacterial cell is called F+ strain donor (Male) and if it is attached to bacterial chromosome, the cell is called Hfr (High frequency of recombination) strain donor (Super male).

During the conjugation between F+– (male) and F (female) strains, the two bacterial cells come close to each other in pair. The F+ cell sends sex pilus which gets attached to F cell forming a conjugation bridge between them. The F-factor then divide into two, out of which one remains in the donor cell and the other migrates into recipient cell through the conjugation bridge. As a result, the F cell now becomes F+ cell. Thus, a conjugation between F+


and F strains always yields F+ progeny. (F + + F ® F+ ).

During the conjugation between Hfr (donor) and F(recipient), the two come close to each other forming a pair. The sex pilus develops from Hfr and gets attached to the wall of F cell. The common wall dissolves and a conjugation bridge is established. The chromosome of Hfr breaks at one point and both the strands of broken end begin to replicate. The chromosome of Hfr becomes linear and have a directional orientation so that the daughter DNA moves into F cell through the conjugation bridge. The migration of DNA into F is such that the F-factor is last to enter. Sometimes complete transfer of DNA from Hfr to F is interrupted due to repture at some point, called R-point. Since complete transfer of DNA occurs only rarely, the F- factor does not usually enter the F and the resulting zygote is not converted to F+. Thus, the newly formed zygote receives only those genes from Hfr which have been transferred during conjugation.

F+ (male)       ×       F(female)


Bacterial genome          F(fertility) Factor


F+(male)                    F+(male)

Fig : Conjugation between F+ male and F female




Sex-duction : As stated earlier the F-factor (fertility factor or sex-factor) remains free in the cytoplasm of F+ strain donor cell and remains attached to bacterial chromosome in Hfr strain donor cell. Sometimes the F-factor gets detached from the bacterial chromosome of Hfr strain cell and resumes an independent status inside the cytoplasm. During faulty separation from the chromosome of Hfr, the F-factor sometimes carries away a small portion of chromosomal DNA along with it. Such F-factor with extra DNA, when transferred from donor to recipient cell during conjugation, becomes part and parcel of recipient chromosome making it heterozygous (or partial diploid). This process is called Sex-duction.

Such F-factor with extra DNA, when transferred from donor to recipient chromosome making it heterozygous (or partial diploid). This process is called conjugation. The bacterium which shows genetic recombination after conjugation is called “Merozygote”.

  • Respiration in bacteria : With respect to oxygen requirement and mode of cellular respiration, bacteria distinctly belong to two broad categories- (i) aerobic and (ii) anaerobic. These are further divided into obligate and facultative types. thus, the bacteria can be grouped into four general categories on the basis of their oxygen

(i)  Aerobic respiration

  • Obligate aerobes : These bacteria grow exclusively in presence of molecular oxygen and fail to survive in its absence, g., Bacillus subtilis, Azotobactor, Arthrobactor, Mycobacterium, etc.
  • Facultative anaerobes : The aerobic bacteria which can also survive in absence of oxygen, g., Aerobacter, Klebsiella, Pseudomonas, etc.

(ii)  Anaerobic respiration

  • Obligate anaerobes : These bacteria grow and multiply in the absence of free They fail to survive under aerobic conditions, e.g., Clostridium botulinum.
  • Facultative aerobes : The anaerobic bacteria which can also survive in presence of oxygen, g., Chlorobium limicola.
  • Mode of nutrition in bacteria : On the basis of mode of nutrition, bacteria are grouped into two broad First is autotrophic and second is heterotrophic bacteria.

Autotrophic bacteria : These bacteria are able to synthesis their own food from inorganic substances, as green plants do. Their carbon is derived from carbon dioxide. The hydrogen needed to reduce carbon to organic form comes from sources such as atmospheric H2, H2S or NH3. These are divided into two categories.

  • Photoautotrophic bacteria : These bacteria are mostly anaerobic They use sunlight as source of energy to synthesize food. But unlike other type of photosynthesis as found in eukaryotic cells, they do not “split water” to transfer energy or to obtain reducing power. Instead they split hydrogen sulphide, thiosulphate, hydrogen or some other organic compound and oxygen is not evolved as a byproduct. They possess a pigment called bacteriochlorophyll which is different from the chlorophyll pigment found in higher plants. This is known as anoxygenic photosynthesis. e.g. Green sulphur (thiothrix) and purple sulphur (chromatiun) bacteria. They can perform photosynthesis in far-red light. Rhodospirillum bacteria fixes CO2 into carbohydrate (Photoautotrophic).





The green sulphur bacteria such as Chlorobium sp. and Chloropseudomonas sp., contain the pigment bacterioviridin (similar to chlorophyll) and thrive well in illuminated environments. These bacteria produce chemical sulphur by removing hydrogen from hydrogen sulphide.

6CO2  + 12H2S ¾¾Lig¾ht ® C6 H12 O6  + 12S + 6H 2 O + Energy

The purple sulphur bacteria such as Thiospirillum sp. and Chromatium sp., contain the pigments bacteriochlorophyll and carotenoids. Theses bacteria utilize inorganic sulphur compounds, selenium compounds or molecular hydrogen.

6CO2  + 15H 2 O + 3Na2 S2 O3  ¾¾Lig¾ht ® C6 H12 O6  + 6H 2 O + 6 NaHSO4  + Energy

Propyl alcohol


The purple non-sulphur bacteria posses the pigment bacteriochlorophyll and accomplish photoreduction of carbondioxide in presence of alcohol, organic acids, etc., e.g., Rhodospirillum sp. Rhodomicrobium sp and Rhodopseudomonas sp.

6CO2  + 12CH3 CHOHCH3  ¾¾Lig¾ht ® C6 H12 O6  + 12CH3 COCH3  + 6H 2 O

The photoautotrophic bacteria thrive well below the surface of lakes and ponds where oxygen content is low and reduced sulphur or other compounds are available.

  • Chemoautotrophic bacteria : Some bacteria manufacture organic matter form inorganic raw materials (such as carbon dioxide) and utilize energy liberated by oxidation of inorganic substances present in the external medium such as ammonia, ferrous ion, nitrates, nitrites, molecular hydrogen, etc. The energy liberated from exergonic chemical reactions is trapped in the ATP molecules which is used in carbon assimilation to synthesize organic matter. There are several types of chemoautotrophic bacteria which are commonly named after the chemical compound they use as source of
  • Sulphur bacteria : These bacteria derive energy by oxidizing hydrogen sulphide or molecular


Beggiatoa, a colourless sulphur bacterium oxidises hydrogen sulphide

(H 2 S)

to water and sulphur. The energy


released is used up and the sulphur granules are deposited inside or outside the body of bacterial cell.

2HS + O2 ¾¾® 2HO + 2S + Energy

The elemental sulphur is oxidized to sulphuric acid by denitrifying sulphur bacteria (e.g., thiobacillus denitrifying) and the energy released during the process is utilized in reproduction, growth and synthesis of other chemical substances.

2S + 2HO + 3O2 ¾¾® 2HSO4 + Energy

These bacteria usually live at the mid-oceanicridge system (2.5 km below sea level). They generally live both freely and with in the bodies of giant tube worms. They can even survive under extremely acidic conditions. Examples of sulphur bacteria are- Beggiatoa, Thiobacillus, Thiothrix etc. They participate in the sulphur cycle in nature.

  • Iron bacteria : Some chemoautotrophic bacteria such as Gallionella, Sphaerotilus, Ferrobacillus, etc, inhabit the environments where irons to ferric form. The Ferric ions are deposited in the form of soluble Ferric hydroxide and the energy released during the conversion is used in the production of




4 FeCO3 + O2 + 6HO ¾¾® 4 Fe(OH)3 + 4CO2 + Energy ( .

  • Hydrogen bacteria : These bacteria utilize free molecular hydrogen and oxidize to hydrogen into water with the help of either oxygen or oxidize salts g. Hydrogenomonas. 2H 2 + O2 ® 2H 2 O + Energy (56 kcal).
  • Amonifying bacteria : They oxidise protein and amino acid into NH3 (amonia). g. Proteus vulgaris,

Bacillus mycoids.

  • Nitrifying bacteria : They oxidise ammonia to nitrites and then into

NH 3  + O2  ¾¾Nitr¾oso¾mon¾as ® NO2  + H 2 O + Energy   &  2NO2  + O2  ¾¾Nitr¾oba¾ct¾er ® 2NO3  + Energy .

  • Denitrifying bacteria : They change nitrogen compound into molecular nitrogen. So that they reduce fertility of soil g. Micrococcus denitrificans, Pseudomonas denitrificans.
  • Methane bacteria : The bacterium Methanomonas utilizes methane as source of carbon and

CH4 + 2O2 ¾¾® CO2 + 2HO + Energy .

  • Methane producing bacteria : These are spherical or rod shaped bacteria which produce methane (CH4 ) from hydrogen gas and carbon dioxide g. Methanobacterium.

CO2 + 4 H 2 ¾¾® CH4 + 2H 2 O .

The synthesis of ATP and reduction of carbon dioxide are linked reactions and used as sources of energy by methanogens (e.g. Methanobacterium). Methane (swamp gas) is produced under anaerobic conditions and can be used as a “biogas”, otherwise it is a pollutant that contributes to the green house effect and global warming.

  • Carbon bacteria : These bacteria oxidize carbon monoxide into carbon dioxide and use the liberated energy, g., Bacillus oligocarbophilus.

2CO2 + O2 ¾¾® 2CO + Energy

  • Heterotrophic bacteria : Most of the bacteria can not synthesize their own orgainc food. They are dependent on external organic materials and require atleast one orgainc compound as a source of carbon of their growth and energy. Such bacteria are called heterotrophic bacteria. Heterotrophic bacteria are of three types. Parasites, Saprotrophs and
  • Parasites : They obtain their organic food or special organic compounds required for their growth from living cells of plants and Some parasitic bacteria are relatively harmless and nonpathogenic, i.e., do not produce disease in hosts. However, majority of parasitic bacteria are pathogenic and cause serious diseases in plants and animals either by exploiting them or by secreting poisonous substances called toxins. Parasites contain several chemical substances (i.e., enzymes, toxins and growth substances) to establish themselves in the host tissues. Some of these chemicals are – agressins (to breakdown connective tissue), leucocides (to kill host phagocytes), streptokinase (to prevent blood clotting) and cellulase (to digest cellulose).

Some examples of pathogenic parasitic bacteria which cause human diseases are

  • Paratyphoid – Salmonella paratyphi
  • Gastroenteritis – Salmonella and Escherichia coli
  • Dysentery – Shiegella dysenteriae, Sonnei, S. Boydii




  • Tularaemia (infected lymph nodes) – Francisella tularensis
  • Influenza– Haemophilus influenzae
  • Botulism (acute food poisoning) – Clostridium botulinum

Many pathogenic bacteria cause destructive diseases of economically important plants. The usual symptoms of bacterial plant diseases are leafspots, rot, blight, wilt, gummosis, canker, scab and crown galls. Some of the common plant diseases caused by bacteria are listed below.

  • Black chaff of wheat caused by Xanthomonas translucens.
  • Wilt of maize caused by Xanthomonas stewartii.
  • Gummosis of sugarcane caused by Xanthomonas asculorum.
  • Red stripe of sugarcane caused by Pseudomonas rubrilineans.
  • Ring rot of potato caused by Corynebacterium.
  • Canker of tomato caused by Corynebacterium michiganense.
  • Leaf spot of Lady’s finger caused by Xanthomonas esculenti.
  • Hairy rot of apple caused by Agrobacterium rhizogenes.
  • Black knot of grapes caused by Pseudomonas tumefaciens.
  • Saprotrophic bacteria : These bacteria obtain their nutritional requirements from dead organic matter (such as animal excreta, corpses, fallen leaves, bread, fruits, vegetables, jams, jellies, etc.). These bacteria breakdown the complex organic matter into simple soluble forms by secreting exogenous digestive enzymes. Then they absorb the simple nutrient molecules and assimilate During assimilation, the bacterial cells oxidize the organic matter to obtain the energy.

Aerobic break down of organic matter is called decomposition or decay. It is usually complete and not accompanied by the release of foul gases. On the other hand, break down of organic matter in absence of oxygen is not always complete and is accompanied by release of foul smell. Anaerobic break down of carbohydrates is usually called fermentation whereas that of proteins is called putrifaction. During putrifaction, the putrifying bacteria cause degradation of proteins in absence of oxygen and convert them into simple ammonium compounds accompanied by evolution of foul gases (hydrogen sulphide, methane, ammonia).

The decomposition caused by bacteria plays very important role in nature by recycling the matter in ecosystems. It also provides inorganic molecules to photosynthesizing orgainsms. The decaying property of bacteria is also used in ripening of cheese, ‘curing’ of tobacco and ‘retting’ of flax. Some free living bacteria (e.g., Azotobacter, Clostridium, Aerobactor, etc.) fix atmospheric nitrogen and improve the fertility of soil.

  • Symbiotic bacteria : Symbiosis is the phenomenon in which the two orgianisms live in close association in such a way that both the partners get mutual benefit from this association. For example, a very well known nitrogen fixing bacteria – Rhizobium forms a symbiotic association with roots of leguminous plants (soyabean, clover, alfalfa, etc.). Producing root nodules. These bacteria reside inside the nodules and reduce atmospheric nitrogen (N 2 ) to The fixed nitrogen is taken up by the plant. In return, the plant provides nutrients and

protection to bacteria.





Another example of symbiosis is the presence of enteric bacterium Escherichia coli (E. Coli) in human intestine. The bacteria shares our food but at the same time checks the growth of harmful putrefying bacteria and releases vitamins K and B12 which help to produce blood components. Similarly cellulose degrading bacteria present

in the stomach of cows and goats help these animals in digesting grasses. In return, they get their nutritional requirements.

Table of nutrition of Bacteria















Green sulphure bacteria (Chlorobium)



Purple sulphure bacteria (Chromatium)


Purple non-sulphure bacteria (Rhodospirillum)













Nutrition       in bacteria














Nitrifying bacteria (Nitrosomonas, Nitrobacter)


Iron bacteria (Ferrobacillus)


Sulphur bacteria (Thiobacillus)


Hydrogen bacteria (Hydrogenomonas)












(Streptococcus, Clostridium)


Saprophytes (Bacillus mycoides)


Methane bacteria (Methanococcus)



(Rhizobium leguminosarum)


  • Spirochaetes : These are free inhabitants of mud and water, and are chemoheterotrophic bacteria. These are spiral or helicoid in shape, covered by flexible cell wall and swim actively with flagella present at both poles or ends. Many diseases are caused by them as Treponema pollidum causes syphilis, Leptospira causes infectious Jaundice and Berrelia causes relapsing fever. Besides some spirochaetes are found in
  • Archaebacteria : They are present in rumen of cattles. This is simplest and most primitive group of The cell wall of these bacteria is made of polysaccharides and proteins (peptidoglycans and muramic acid are absent in cell wall). Further branched chain lipids are present in plasma membrane of archaebacteria, due to which these can face extremes of conditions of temperature and pH. Archaebacteria are considered to be ‘oldest of living fossils‘. Three main groups of archaebacteria are following.





  • Methanogens : These are strict anaerobic bacteria and mainly occur in muddy areas and also in stomach of cattle, where cellulose is fermented by microbes. These are responsible for methane gas (CH 4 ) formation in bio- gas plants, because they have capacity to produce CH4 from CO2 or formic acid (HCOOH).
  • Salt lovers archaebacteria or halophiles : These are also anaerobic bacteria, which occur in extreme saline or salty conditions (upto 35% of salt or NaCl in culture medium). A purple pigmented membrane containing bacteriorhodopsin is developed in sun-light in these bacteria, which utilizes light energy for metabolic activities (different from photosynthesis).
  • Thermoacidophiles : These are the bacteria which are found in hot sulphur springs (upto 80oC). As against first two groups of archaebacteria, these are aerobic These have the capacity to oxidize sulphur to



at high temperature and high acidity (i.e. pH 2.0), hence given the name thermoacidophiles, i.e.,


temperature and acid loving. Some of these bacteria are able to reduce sulphur to conditions.


under anaerobic


  • Actinomycetes : It is a group of unicellular branched filamentous bacteria which resemble fungal They grow in the form of radiating colonies in cultures and therefore, commonly called ray fungi. They are Gram +ve chemo-organotrophic, saprotrophic bacteria. Most species are


facultative anaerobic. These are filamentous bacteria (like moulds or fungi). These are generally present as decomposers in soil. These occur most commonly and abundantly in soil, fresh water, manure, food products etc. The filaments are aseptate (non-septate) branched and very thin (about 0.2 to 1.2 mm in width). The wall contains mycolic acid. They reproduce asexually by means of conidia which produced at tips of filaments. The endospores are not formed. Most of these secrete chemical substances having antimicrobial activities called antibiotics.

Some of the most common and affective antibiotics are obtained from the different species of the genus streptomyces. For example – Streptomycin (from S. griseus), Chloromycetin (from S.venezuelae), Terramycin (from S. rimosus), Aureomycin (from S.aureofaciens), Erythromycin (from S.

Chain of conidia















Fig : Actinomycetes : A mycelium of streptomyces bearing conidia


erythreus), Neomycin (from S. fradiae), Carbomycin (from S. halstedii), Amphotericin B (from S. nodosus), etc.

Some species are pathogenic and cause diseases, e.g. Mycobacterium. Some common diseases in plants are yellow ear rot of wheat (Tondu disease) caused by corynebacteriom tritici and scab of potato by streptomyces scabies.

(i)  Diseases in human beings

  • Tuberculosis is caused by Mycobacterium tuberculosis
  • Leprosy is caused by Mycobacterium
  • Buruli’s ulcer is caused by Mycobacterium
  • Actinomycosis is caused by Actinomyces
  • Diphtheria is caused by Corynebacterium





  • Animal diseases
  • Tuberculosis of cattle is caused by Mycobacterium
  • Lumpy jaw is caused by Actinomyces
Note :
  • Zoogloea stage : The bacterial cells often become attached from end to end forming long filamentous chain which are embedded in a mass of mucilage forming a scum layer on It is called as Zoogloea stage.
  • Rickettsias (Ricketts 1909) : They are Gram negative obligate pleomorphic but walled intercellular parasites which are transmissible from arthropods. They are intermediate between true bacteria and viruses. Rickettsiae require exogenous factors for Cell wall is like typical bacterial wall. ATP synthesis is absent but ADP is exchanged with host cell ATP. They have genome and size (0.3-0.5 mm ) smaller than true bacteria but have

a longer generation time. Internally the cells of rickettsias contain DNA as well as RNA in a ratio of 1 : 3 : 5. The cell walls contain muramic acid and are sensitive to lysozyme. Flagella, pilli and capsule are absent reproduction occurs by binary fission. The natural habitat of rickettsiae is in the cells of arthropod gut. they cause typhus group of fevers. Spread by droplet method, lice, ticks, fleas, etc.

(i) Diseases in human beings

  • Typhus fever is caused by Rickettsia
  • Rocky mountain spotted fever is caused by Rickettsia rick
  • Q fever is caused by Coxiella
  • Scrub typhus is caused by Rickettsia
  • Importance of bacteria : Bacteria are our ‘friends and foes’ as they have both useful and harmful

Useful activities

  • Decay of organic wastes : Many saprotrophic bacteria act as natural scavengers by continuously removing the harmful organic wastes (e., dead remains of animals and plants) from man’s environment. They decompose the organic matter by putrifaction and decay. The simple compounds produced as a result of decomposition and decay (viz., carbon dioxide, carbon monoxide, nitrates, sulphates, phosphates, ammonia, etc.) are either released back into the environment for recycling or absorbed by the plants as food. Thus, the bacteria play duel role by disposing of the dead bodies and wastes of organisms and by increasing the fertility of soil.
  • Role in improving soil fertility : Saprotrophic bacteria present in soil perform various activities for their Some of these activities improve the fertility of soil by formation of humus, manure, etc.
  • Humus : The microbial decomposition of organic matter and mineralization results in the formation of complex amorphous substance called humus. The humus improves the aeration, water holding capacity, solubility of soil minerals, oxidation-reduction potential and buffering capacity of the
  • Composting : It is conversion of farm refuse, dung and other organic wastes into manure by the activity of saprotrophic bacteria (g., Bacillus stearothermophilus, Clostridium thermocellum, Thermomonospora spp, etc.)


  • Adding sulphates : A few sulphur bacteria (g., Beggiatoa) add sulphur into the soil by converting into sulphates.

H 2 S





  • Role in nitrogen cycle : Nitrogen cycle existing in nature, comprises of –
  • Nitrogen fixation : Many free-living soil inhabiting bacteria such as, Azotobacter (aerobic), Clostridium (anaerobic), etc. have ability to fix atmospheric nitrogen into ammonia. The other group of nitrogen fixing bacteria live in symbiotic association with other The most important symbiotic nitrogen fixing bacteria is Rhizobium spp. The various species of Rhizobium inhabit different leguminous plants. For example, R. leguminosarium infects soyabeans, etc. They develop root nodules and fix atmospheric nitrogen into ammonia in symbiotic association with leguminous plants. The fixed nitrogen is partly taken up by the leguminous plants and metabolised. A part of fixed nitrogen is diffused out into the surrounding soil.
  • Ammonification : The nitrogenous compounds of the dead remains of plants, animals and their excretory products are decomposed into ammonia by a number of bacteria and other microorganisms. The conversion of nitrogenous organic compounds into ammonia is termed as It is carried by many ammonifying bacteria such as Bacillus ramosus, B. vulgaris, B. mycoides, etc.
  • Nitrification : Many bacteria enhance the nitrogen fertility of soil by converting ammonium compounds to nitrites (g., Nitrosomonas) and nitrites into nitrates (e.g., Nitrobacter).

The Nitrosomonas group oxidizes ammonia into nitrite –



NH + + 3 / 2 O2

¾¾® NO + H

2O + H + + Energy ­



The Nitrobacter group oxidizes nitrite to nitrates –


NO + 1 / 2 O2 ¾¾® NO + Energy ­

  • Denitrification : The nitrates and ammonia are converted to nitrous oxide and finally to nitrogen gas by several denitrifying bacteria, g., Pseudomonas fluorescence, P. denitrificans, Bacillus subtilis, Thiobacillus denitirficans, etc.
  • Sewage, disposal : Ability of anaerobic bacteria to purify the organic matter is used in the the sewage disposal system of cities. The faeces are stored in covered reservoirs and allowed to purify. The solid matter is decomposed into liquidy sludge which is passed through coarse The effluent is finally purified and drained out into the river or used as fertilizer in the fields. The common bacteria involved in sewage disposal are – Coliforms (E. coli), Streptococci, Clostridium, Micrococcus, Proteus, Pseudomonas, Lactobacillus, etc.
  • Role in Industry : Useful activities of various bacteria are employed in the production of a number of industrial Some of these are given below –
  • Lactic acid : Lactic acid is commercially produced from pasteurized whey (the watery part of milk) through fermentation caused by Lactobacilus bulgaricus and delbrueckii.
  • Curd : Curd is prepared from pasteurized milk by the process called curdling. It is initiated by adding a starter culture of Lactobacillus bulgaricus and Streptococcus thermophillus, into the milk at 40°C. Lactobacillus converts lactose to lactic acid whereas Streptococcus causes coagulation of casein due to
  • Cheese : Preparation of cheese from the milk involves two main steps – first curdling of milk, and second the subsequent ripening of solid curd by the use of different bacterial
  • Butter : It is prepared by churning of sweet or sour cream. The microorganisms responsible for preparation of butter cream are – Streptococcus lactis and Leuconostoc citrivorumare. The characteristic butter




aroma develops due to a volatile substance – diacetyl. It is produced by the action of streptococcus on pasteurized milk.

  • Retting process : Fibres of flax, hemp and jute are separated by the process called retting. During this process the stems of the plants are submerged in water, where the bacterial activity results in the rotting of softer The tough bast fibres become loosened and easily separated from each other. These fibres are spun and woven into various articles.
  • Vinegar : Country made vinegar is a fermentation product of cane juice, molasses or fruit juices. It is produced in two steps – first conversion of sugars into alcohols by alcholic fermentation carried by yeast, and the second, conversion of alcohol to acetic acid by the action of bacteria Acetobacter ( orieansis, A. acetic, A. schuizenbachi, etc.). Vinegar is used in the preparation of pickles or in place of acetic acid. It is used as preservative of meats and vegetables.
  • Role of bacteria in human being : coli (gram-ve) bacteria live in colon region of intestine of man and other animals and play an important role in digestion process.

(vii)  Medicinal uses

  • Vitamins : Production of riboflavin (vitamin B2) involves the activity of bacterium – Clostridium butyticum. The well known vitamin C (ascorbic acid) is produced from sorbital by the action of Acetobactor
  • Serum and vaccines : Many bacteria are used in the preparation of serums and vaccines. These substances induce immunity to various diseases in man. Serums are effective against certain diseases like diphtheria, pneumonia, , whereas the vaccines are effective against typhoid, smallpox, cholera, etc.
  • Enzymes : Some bacteria live in the alimentary canal of herbivorous animals like cow, horse, goat, etc. and help in the production of certain enzymes which digest the The enzymes proteases are produced by bacteria Bacillus subtilis. Similarly, the enzyme pectinase is produced by Clostridium sp, which is used in retting of flax.
  • Antibiotics : These are the chemical substances produces by living microorganisms capable of inhibiting or destroying other microbes. These are the products of secondary and minor metabolic pathways, mostly secreted extracellularly by the These are used in controlling various infectious diseases.

At present more than 5000 antibiotic substances are known and approximately 100 are available for medicinal use. The most important bacterium which produces maximum number of antibiotics is Streptomyces.

A list of some common antibiotics, their sources and their applications.


S. No.AntibioticObtained fromUsed against
AStreptomycinStreptomyces griseusGram-positive and Gram-negative bacteria, TB, tularemia (rabbit fever), influenza, meaningitis, baciltary dysentery, etc.
BActidineS. griseusPlant diseases caused by fungi.
CChloromycetinS. venezuelaeGram-positive and Gram- negative bacteria, typhoid, rickettsias
DTetracyclineS. aurefaciensGram-positive and Gram-negative bacteria, rickettsiae.
ETerramycinS. ramosusGram positive and Gram-negative bacteria.
FErythromycinS. erythreusGram positive bacteria, whooping cough, diphtheria.




GNeomycinS. fradiaeGram- positive, Gram negative and TB bacteria.
HAmphomycinS. carusGram-positive bacteria,
IAmphotericin BS. nodosusYeast, fungi
JLeucomycinS. kitasoensisGram-positive bacteria.
KTrichomycinS. hachijoensisYeast and fungi.
LViomycinS. floridaeGram-positive, Gram-negative and TB bacteria.
MBacitracinBacillus subtilisGram-positive bacteria
NGramicidinB. brevisGram-positive bacteria.
OTyrothricinB. brevisGram-positive and Gram-negative bacteria.
PPolymyxin BAerobacillus polymyxaGram-negative bacteria.


Harmful activities

  • Food poisoning : Some saprotrophic bacteria cause decay of our food, e., they alter their normal form and induce unpleasant aroma, taste and appearance. Some bacteria produce powerful toxins in food to cause “food poisoning”. Consumption of such food may cause serious illness or even death. Symptoms and causal organism of some important types of bacterial food poisoning are listed below :–
  • Botulism : It is caused by Clostridium The main symptoms are vomitting followed by paralysis and death.
  • Perfringens poisoning : It is caused by Clostridium perfringens. Symptoms appear in the form of diarrhoea and acute abdominal
  • Staphylococcal food poisoning : It is caused by Staphylococcus aureus. Common symptoms are nausea, vomitting and
  • Spoilage of food : Some examples of bacterial food spoilage are listed below :–
  • Salmonellosis in poultry and eggs is caused by Salmonella.
  • Red rot of eggs is caused by Serratia
  • Greening on meat surface is caused by Lactobacillus and Leuconostoc.
  • Black rot of eggs is caused by
  • Green rot of eggs is caused by Pseudomonas.
  • Souring of milk is caused by Lactobacillus and Streptococcus.
  • Explosion of curd (gas production) is caused by Clostridium and Coliform
  • Ropiness (e., slimy milk) is caused by Klebsiella sp. Enterobacter spp.
  • Pollution of water : There are reports of epidemics of cholera, typhoid, jaundice and other infectious diseases, which were caused by polluted Many pathogenic bacteria such as, Vibrio cholerae, Salmonella typhi, Leptospira cetero-haemorrhagiae, etc. pollute water and make it unfit for drinking. These are eliminated by chlorination.
  • Deterioration of textiles : Some bacteria (g. Cytophaga, Vibrio and Cellulomonas) damage cellulose of textiles.
  • Abortion :Bacteria like salmonella induce abortion in goats, horses, sheep
  • Biological warfare : Some bacteria which cause diseases like anthrax, black- leg, tuberculosis etc, are employed as secret war
  • Denitrification : Denitrification bacteria like bacillus licheniformis, Pseudomonas aeruginosa convert nitrates and nitrites into free nitrogen, thus responsible for the process of Thus soil is depleted of essential nutrient like usable form of nitrogen.
  • Putrefaction : It is the spoilage of protein in the absence of O2 by the putrefying bacteria g., Proteus, Mycoides.
  • Retting of fibres : It is the hydrolysis of pectic substances that bind the cells




  • Diseases : Bacteria are the causative agents of a large number of human diseases such as pneumonia, typhoid, dysentery, cholera, plague, influenza, tetanus, diphtheria, tuberculosis, leprosy, syphilis, whooping cough They are also responsible for several plant diseases and animal diseases.

Most of the pathogenic bacteria are Gram – ve, rod-shaped (Bacillus) and non-spore forming.


Name of DiseaseBacteria
(a) In human beings 
PneumoniaDiplococcus pneumoniae
TyphoidSalmonella typhosa
CholeraVibrio cholerae
Plague (Black death)Pasteurella pestis
MeningitisNeisseria meningitides
GonorrhoeaNeisseria gonorrhoeae
SyphillisTreponema pallidum
DiorrhoeaBacillus coli
DiptheriaCorynebacterium diptheriae
TuberculosisMycobacterium tuberculosis
GangarinClostridium perfringens
JaundiceLeptospira ictero haemorrhagae
Whooping coughHaemophilus pertussis or Bordetella pertussis
Tetanus (lockjaw)Clostridium tetani
Bacterial dysentryShiegella dysentriae
LeprosyMycobacterium leprae
(b) In animals 
AnthraxBacillus anthracis
Black leg diseaseClostridium chauvi
(c) In plants 
Soft rot of potatoPseudomonas solanacearum
Citrus cankerXanthomonas citri
Bacterial blight of paddyXanthomonas oryzae
Tundu disease in wheatCorynebacterium tritici
Potato wiltPseudomonas solanacearum
Fire blight of apple and peachErwinia amylovora
Crown gall of sugar beetAgrobacterium tumefaciens
Black rot of cabbageXanthomonas compestris


Important Tips

  • Robert Koch (1881) is the father of modern
  • Bacteria are studied under
  • Mycobacterium leprae is exception of koch’s postulate because it can not grow in culture
  • Bacteria are unicellular They were first seen by a Dutch lens marker, Anton Von Leeuwenhock (1683).
  • Bacteria differ from animals in having a rigid cell




A scientist named Gram, stained the bacteria with crystal violet and Iodine solution.

After washing them with acetone or alcohol Gram+ bacteria retain deep violet or purple colour. Bacterial cell wall is made up by peptidoglycans and muramic acid.

Father of modern antiseptic surgery. Joseph Lister.

Insulin is the first hormone which obtained from genetically engineered bacteria. Free living N2 fixing bacteria – Azatobacter and polymyxa.

Clostridium butyticum has been used in the synthesis of vitamin B.

Commonsals : Those microorganism which are living in large intestine of human and that feed on undigested food without harming the host are termed as.

By the hanging drop slide we can see movement of bacteria. Capsule is made up by polysaccharides and polypeptides.

Mesosomes contain oxidative enzymes of electrons transport system. It is the folding of plasmamembrane which also help in respiration so they are called chondriods.

External DNA enters in bacteria through mesosomes.

In the bacterial cystoplasm, membranous organelles like (mitochondria, chloroplast, ribosome, endoplasmic, reticulum, Golgy body, etc.) are absent.

In bacteria flagella may be present, PS II absent, photosynthesis is a nonoxygenic.

In the cytoplasm 70 S ribosomes present. Bacteria also contain fats, glycogen, protein and photosynthetic pigment (carotenoids). True nucleus is absent in them. Their nucleus is called nucleoid.

Histone proteins are absent in bacterial cell (Prokaryotic cell). Flagella of salmonella bacteria contain H-antigens.

Bacterial flagella is composed of 1 or 3 tubuline fibril organisation against 9+2 fibril organisation in eukaryotes. Bacteria – have two important factors located on plasmid. (1) F.factor (sex factor). (2) R-factor (resistance factor).

Gram +ve bacteria (Bacillus and Clostridium) produce resting spores called endospores which are formed in unfavourable conditions. Transformation process is reported by Griffiths (1928) in mice.

Polyribosome always attached with m RNA.

Volutin granules are the source of energy in bacteria.

Transduction was first reported by Zinder and Lederberg in (1952). In this process DNA of a bacterial cell transfer to another bacterial cell by Bacteriophage.

Conjugation – was discovered by Lederberg and Tatum. In this process two different types of bacteria are connected by conjugation. The bacterium which contains F-factor is called F+/donor, the other bacterium which lacks this factor is called F or the recepient.

Iron bacteria – they oxidise ferrous compound to ferric forms eg. Thiobacillus.

Chemoautotrophic bacteria – these bacteria oxidise a number of inorganic compounds to obtain energy for the assimilation of CO2. they cannot make use of light energy.

Escherichia coli (E. coli) is a facultative aerobic bacteria found in the colon region of intestine of human being. this bacterium was discovered by Escherisch (German scientist).

Chemically E.coli has about 70% H2O, 15% proteins, 6% RNA, 1% DNA. 2% Lipid, 3% carbohydrates, etc.

Rhizobium bacteria found in a symbiotic relationship with leguminous plant. They fix N2 in root nodule from atmosphere. It has nifgene. Non symbiotic anaerobic non-photosynthetic N2 fixing bacteria is clostridium.

Azotobacter is found freely in the soil as saprophyte. It is estimated that such bacteria are capable of adding 5-25 Kg. of nitrogen per acre per year.

Nitrifing bacteria transform NH 3 into nitrates.

Bacterial size ranges between 2 – 5m , Smallest bacterium – Dialister pneumosintes. Largest bacteria are – Spirillum laid.





  • Plasmid is the extra DNA They can independently replicate. Plasmid are not essential for normal life process.
  • Eukaryotic flagella is formed of tubulin protein while bacterial flagella is made up of flagellin protein.



Mycoplasmas were discovered by E. Nocard and E. R Roux (1898). They were first isolated from bovine sheep


suffering from pleuropneumonia. They are often designated as pleuropneumonia–like organisms (PPLO). These organisms were later put under the generic name mycoplasma by Nowak (1929). In 1966 international commitee of Nomanclature of bacteria, placed mycoplasmas under the class mollicutes, which consists of two genera Mycoplasma and Acholeplasma. These are the simplest unicellular non- motile known aerobic prokaryotes without cell wall. So that they can change their shape therefore called Jockers of microbiological park. They are considered to be intermediate between bacteria and viruses. They have smallest living cells of prokaryotes. They are known to cause a number of diseases in human beings, animals and in plants. Mycoplasma can grow out side the host cell. Thus it is clear that mycoplasma are not obligate parasites like viruses. They are gram-negative.

Lipoprotein membrane (3 Layers)







Soluble protein





Soluble RNA


Fig : Mycoplasma–showing structural details


  • Distribution : Mycoplasmas occur in soil, sewage water, different substrates, and in human beings, animals and plants. They have also been found in hot water springs and other thermal environments. They are the frequent contaminants in tissue cultures rich in organic
  • Structure : They are one of simplest prokaryotic Their size varies from 0.1 – 0.15 mm. They lack the cell wall. Due to the absence of the cell wall, these organisms are highly elastic and readily change their shape; hence the mycoplasmas are irregular and quite variable in shape. This nature is called pleomorphism. They may be coccoid, granular, pear–shaped, cluster–like or filamentous. Mycoplasma cells are covered with three layered plasma memberane. Unit membrane is made up of lipoprotein. Normally there are no mesosomes but in stationary phase, some time mesosomes are also present on plasma membrane. They lack the well organised nucleus, endoplasmic reticulum, mitochondria, plastids, golgi bodies, centrioles, flagella, etc. The genetic material is present in the form of a nucleoid. The latter consists of a single, circular, double–stranded molecule of DNA, without a nuclear envelope. Unlike other prokaryotes, it is coiled throughout the cytoplasm. The cytoplasm contains the ribosomes which are 70S. It also contains RNA, proteins, lipids and many kinds of enzymes used in biosynthetic reactions. Lipids include cholestrol and cholestrol esters which are characteristic of animal cells and are not found in true bacteria and cyanobacteria. The amount of DNA and RNA in the cells is usually less than half of that which occurs in other prokaryotes. There is 4% DNA and 8% RNA. It is perhaps the lowest limit required for a cellular organism. Sterol is must compulsary for growth of mycoplasma. Mycoplasmas are Gram–negative.
  • Physiology and reproduction : Mycoplasmas are usually non–motile. They are sensitive to tetracycline and resistant to These are destroyed usually by treatment of heat at 50o C for 6 hours mycoplasma are osmotically inactive. Some forms, show gliding movements. Mycoplasmas are heterotrophic in their mode of nutrition. Some of them are saprotrophs, but most of them are parasitic on plants and animals including man. They




reproduce by budding or binary fission. Fragmentation specially in filamentous forms. Besides this, Mycoplasma reproduces by elementary cell bodies also. It is also called baby particle. It is a kind of vegetative reproduction.

Culture of mycoplasma : These can be cultured in non-living medium, although they grow well in living medium, contain chick tissue. In non-living medium, they require agar-agar and blood serum.

  • Economic importance : Mycoplasma cause serious diseases in human beings, animals and plants. Some of these are given
    • Diseases in Human beings : Mycoplamsa hominis causes pleuropneumonia, inflammation of genitals and endocarditis, Mycoplasma pneumoniae causes primary a typical pneumonia (PAP), haemorrhagic laryngitis, etc. Mycoplasma fermentatus and M. hominis cause infertility in man, otitis media (inflamation of middle ear).
    • Diseases in animals : Mycoplasma mycoides causes pneumonia in Mycoplasma bovigenitalum,

causes inflamination of genitals in animals. Mycoplamsa agalactia causes agalactia of sheep and goat.

  • Diseases in plants : Common Mycoplasmal diseases of plants are: Bunchy top of papaya, witches’ broom of legumes, yellow dwarf of tobacco, stripe disease of sugarcane, little leaf of brinjal, clover phylloidy, big bud of tomoto, Mycoplasma are the smallest organisms which produce diseases in plants.
  • Difference between L-form bacteria and mycoplasma : In the culture of bacteria, some bacterial cells are developed which are without cell wall and such bacterial cells which are without cell wall are known as L-form bacteria (‘L’ for Lister institute, where these were reported).

Important difference between L-form bacteria and mycoplasma is that under optimum nutritional conditions.

L-form bacteria will develop cell walls whereas mycoplasma will never develop cell wall.


Important Tips

  • Mycoplasma is the smallest cell of
  • Mycoplasma are also called Joker of plant
  • They lack cell They are covered by three layers of plasmamembrane
  • Mycoplasma are sensitive to tetracycline and resistant towards penicillin.
  • Bleb and infra bleb – These are peculiar structures which are formed at both ends during binary fission in mycoplasma and as soon as binary fission is complete, these structures So the significance of bleb and infra bleb is not known.



The new name of cyanobacteria has been given to myxophyceae or cyanophyceae. Cyanobacteria form a group of ancient Gram negative, photosynthetic prokaryotes. Many botanists prefer to call them blue-green algae. They have survived successfully for about 3 billion years. They may cause water blooms.

  • Distribution : Cyanobacteria are predominantly fresh water forms, a few are marine. They are found growing even under extreme situations such as in hot springs and the undersides of icebergs. The fresh water forms occur in ponds, lakes, pools and They impart unpleasant taste and smell to the water. One species of cyanobacteria containing red pigment (Trichodesmium erythracum) flourishes in Red Sea and is responsible for the




red colour of its water. Some grow in the soil and help in fixation of nitrogen and utilize it in metabolism. Nostoc colony is found into the thallus of Anthoceros. Colonies of Nostoc and Anabaena grow in paddy fields. Some live in symbiotic relationship with other organisms. In Oscillatoria filaments show oscillating motion.

  • Organisation of Thallus : The organisation of thallus ranges from unicellular to branched heterotrichous
    • Unicellular forms, g., Chroococcus, etc.
    • Unicellular polar thalli with a definite base and apex, g., Dermocarpa, Chamaesiphon, etc.
    • Multicellular colonial forms, g., Gloeocapsa, Trichodesmium, Merismopedia, Microcystis, etc.
    • Simple unbranched filamentous forms without heterocysts and akinetes, g., Arthrospira, Oscillatoria, Spirulina, Phormidium, Lyngbya, Symploca, Microcoleus, Schizothrix, etc.
    • Simple unbranched filaments with heterocyst, g., Nostoc, Anabaena, Aulosira, Anabaenopsis, Cylindrospermum, etc.
    • Unbranched heterocystous filaments with base and apex, g., Rivularia, Gleotrichia, etc.
    • Heterotrichous filaments with false branching, g., Scytonema, Plectonema, etc.
    • Heterotrichous filaments with true branching, g., Haplosiphon, Stigonema, etc.
  • Oxygen revolution : They are said to be earliest oxygenic Due to their activity atmosphere turned aerobic, thus providing favourable conditions for the evolution of aerobic bacteria and other eukaryotes.
  • Symbiotic forms : There is a long list of cyanobacteria which are found in symbiosis with plants and They may be associated with lichens. Many members are associated with liverworts, mosses, ferns, flowering plants, fungi, protozoa, sponge, shrimp and sometimes a mammal. They have been reported in bryophytes like Anthoceros, Anabaena cycadeae is found in coralloid roots of cycads.
  • Movement of cyanobacteria : Flagella are completely absent but the movement occurs in some genera by special gliding Such movements are connected with the secretion of mucilage. The genus oscillatoria exhibits pendulum like oscillating movement of its anterior region.
  • Ultrastructure : The cyanobacterial cell is normally larger than a bacterial cell. Like a bacterial cell, it consists of a tiny mass of protoplast surrounded by cell wall. It is differentiated into cell wall, cytoplasm and a
    • Cell wall : The cell wall completely surrounds the Cell wall is made up of muramic acid, Lipopolysaccharides, Glucosins, Glutamic acid and a- diaminopimalic acid. It is a thin structure made up of cellulose and peptidoglycan. External to the cell wall is a mucilaginous sheath. It has a great water absorbing and retaining capacity. The sheath is made up of reticulated arranged microfibrils within an amorphous matrix. The fibrils may be composed of pectic acid and mucopolysaccharides, sheath may be thin or thick, hyaline or pigmented, homogenous and stratified. The cell wall is firm and rigid. It is two layered. The outer layer is convoluted and

inner layer is smooth. The inner layer is made up of peptidoglycan similar to bacterial cell wall.

  • Cytoplasm : The cell wall is followed by plasma membrane made up of lipid and Inner contents of the cell can be distinguished into two regions-outer pigmented region called chromatoplasm and central hyaline centroplasm. The membrane bound structures like true mitochondria, chloroplasts, endoplasmic reticulum, golgi bodies, true vacuoles, etc. are absent. The photosynthetic pigments are located in broad sheet like lamellae, called





thylakoids. The thylakoids are restricted to peripheral region of cytoplasm usually arranged in two or more parallel stacks. Some lipid globules occur within the thylakoids whereas phycobilisome particles are attached to their surfaces. Some authors suggest that these lamellae also provide the sites for cellular respiration. The photosynthetic pigments present in the cell are – chlorophyll a, b carotene, Myxoxanthophyll, myxoxanthin, C-phycocyanin and C- phycoerythrin. The C-phycocyanin is blue and C-phycoerythrin is red in colour. If C-phycocyanin is more as compared to C-phycoerythrin, it gives characteristic blue- green colour to the algae. Other cellular inclusions are gas vacuoles, cyanophycin granules, volutin granules, b – granules (lipid droplets), polyhedral bodies, 70s ribosomes,

etc. The gas vacuoles are common in planktonic forms. They are the vesicles filled with gas and bounded by single membrane. They serve to regulate the buoyancy of the planktonic forms. In diffuse or low light intensities, they are large and help to bring the cyanobacteria at the surface. Cyanophycin granules are made up of stored protein. Volutin granules store phosphate, a-granules contain cyanophycean starch, bgranules contain lipid droplats.

  • Nucleoid or genophore : It lacks a definite nucleus. The nuclear material consists of a single chromosome made up of a naked strand of DNA helix which lies in the DNA is not associated with histone proteins. In this respect, they resemble the circular chromosome of bacteria. The nucleolus is absent and the nucleoid is not bounded by a nuclear membrane (This type of nucleus called incipient nucleus).
  • Nutrition : Because of the presence of chlorophyll–a, cyanobacteria synthesis their own food from carbon dioxide and water in the presence of sunlight. Certain cyanobacteria like Nostoc and Anabaena fix atmospheric nitrogen in the presence of oxygen. They are obligate photoautotrophs. They do not grow in darkness. Cyanobacteria are the earliest photosynthesizers which made the earth’s atmosphere aerobic.This provided the suitable condition for the evolution of aerobic bacteria and
  • Reproduction : Cyanobacteria reproduce asexually by fission and fragmentation. Unicellular forms multiply by binary fission flagella are absent in vegetative as well as reproductive The filamentous forms reproduce by fragmentation of their thallus into hormogonia (as in Nostoc and Oscillatoria). Heterocysts and akinites are used in propagation. These serve as vegetative means of

propagation. Except oscillatoriaceae all cyanophycean member contain heterocyst. It is a special type of cell of cynobacteria. Food material, stored in them in the form of cyanophycean starch.

  • Nitrogen fixation in cyanobacteria : Like many bacteria, several forms of blue green algae have the capacity to fix atmospheric nitrogen into nitrogenous compounds. This capacity is restricted to filamentous heterocystous forms like Nostoc, Anabaena, Aulosira, Mastigocladus, scytonema and calothrix. Under anaerobic conditions, some nonheterocystous forms can also fix atmospheric nitrogen (Oscillatoria, Plectonema, Phormidium). This additional capacity of

N 2 – fixation along with CO2 – fixation makes them truely autotrophic


plants. In this sense, they are considered to be largely responsible for the maintenance of soil fertility in tropical and temperate regions. Some

Fig : Nostoc Habit and Heterocysts


species of blue-green algae have a great contribution to increase the fertility of rice fields in tropical countries like India (e.g., Anabaena, Aulosira, Zolypothrix).

Biochemistry and mechanism of nitrogen fixation in blue- green algae have been studied in detail. The radioactive tracer technique and other. Researches have shown that the atmospheric dinitrogen (N º N) is reduced to ammonia in the presence of a reducing agent. The reaction is a stepwise process and is catalyzed by the nitrogen fixing enzyme- nitrogenase utilizing energy. The enzyme nitrogenase works under anaerobic conditions.




The fixed nitrogen can be utilized in its own metabolism by blue-green algae. The nitrogenous compounds come to the soil after death and decay of blue –green algae or by direct leaching of the soluble nitrogenous compounds. In soil, the nitrogenous compounds are available for use by higher plants.

As stated earlier, the enzyme nitrogenase works anaerobic conditions. The thick walled heterocysts provide a suitable anaerobic environment for nitrogenase, even under aerobic conditions. Under anaerobic conditions both heterocystous and non- heterocystous forms can fix nitrogen because of the proper functioning of the nitrogenase. Leghaemoglobin present in leguminous root nodules that act as oxygen scavenger because nitrogenase works under anaerobic condition.

(10)  Economic importance

  • Useful activities
  • Spirulina is cultivated in tanks as a protein rich food for fish and other
  • Some cyanobacteria like Nostoc, Anabaena, Scytonema increase the soil fertility by fixing the free nitrogen of the atmosphere. So that it is used as a biofertilizer.
  • Reclamation of soil : Certain cyanobacteria like Nostoc commune, scytonema ocellantum, Aulosira fertissima are used for reclamation of usar (sterile alkaline) These organisms secrete acidic chemicals which counteract the alkalinity of the usar soil.
  • Food : Nostoc community are as food by Chinese and South Food is called yoyucho.
  • Prevention of growth of mosquito larva : Few species of Anabaena and Aulosira are inoculated in ponds to check the development of mosquito
  • Green manure : In sambhar lake of Rajasthan, Anabaena and Spirulina are produced in large Local people use it as green manure.
  • Soil erosion : Some cyanobacteria, such as Anabaena, Lyngbya help in conservation of soil, thus checking soil erosion.
  • Plant succession : few cyanobacteria located inside lichens help in plant succession due to their growth on barren

(ii)  Harmful activities

  • Spoilage of drinking water : Forms like Anabaena not only spoil the taste of drinking water but also produce toxic
  • Diseases : Skin infections may be caused by cyanobacteria like Lyngbya.
  • Toxin secreting cyanobacteria : They are mainly responsible for water blooms. By the death and decomposition water gets contaminated and unfit for normal use some cyanobacteria like Ribularia release toxins which is harmful for aquatic


Systematic position –

Kingdom          –         Plantae Sub kingdom     –                      Thallophyta





Phylum            –        Cyanophyta

Class               –        Myxophyceae (Cyanophyceae)

Order               –        Nostocales

Family             –        Nostocaceae

Genus              –        Nostoc (Nostoc name is given by Vaucher)

Common Name –       Fallen stars

  • Habitat : This is an alga of both terrestrial and aquatic habitats. Terrestrial species grow commonly either on bare soil or intermingled with plant parts. Sometimes aquatic species submerged lying on the bottom of the pools, or attached to a substratum and some time free floating Nostoc is found in a colony. All colonies are covered by mucilaginous Nostoc associate with fungi to form lichens.

They behave as “space parasites” in the thalli of filamentous Anthoceros.

  • Morphology : The Nostoc plant is filamentous and the trichomes are unbranched and appear Individual cells are mostly spherical but some times barrel shaped or cylindrical also. Single filament of Nostoc without mucilagenous sheath is called Trichome. Trichome with mucilaginous sheath is called filament.


Common sheath             Mucilage




Cell wall        Central body





Cyanophycin granules








Pseudo vacuole A

Polar nodule



Nostoc : species


All the cells of the trichome are similar in structure but at some intervals are found slightly larger rounded light yellowish thick walled cells called as heterocysts it can fix N2. It is formed from normal cell when dim light is present. Trichome mostly breaks near heterocyst and forms hormogonia and thus they help in its multiplication.

The heterocysts are intercalary and posses a very thick outer wall. Each heterocyst is connected with vegtative cells, on two sides through the prominent pores in to the wall. Which later on are occupied by a refractive cyanophycean granule called polar nodule.

Each cell of Nostoc has a primitive nucleus or (prokaryotic).

  • Reproduction : There is no sexual reproduction in Nostoc but it reproduces asexually by the following
  • Hormogonia : The filaments break at number of place into smaller pieces, called as hormogonia. By decay of an ordinary cell they slip out of the mucilaginous sheath and grow into new




  • Resting spores or akinites : Under certain condition some of the vegetative cells enlarge and accumulate food material and develop thick walls. These are called akinites. The akinites germinate after a period of rest and their contents are liberate out through a Protoplast by further division forms the new filament.
  • Heterocysts : In exceptional case heterocyst may become functional and on germination develops a new
  • Endospores : Nostoc microscopium, Nostoc commune.

(14)  Economic importance

  • Many species of Nostoc fix atmospheric nitrogen and thus increase the soil fertility. Heterocysts “the unique structure” of Nostoc filament function as sites for nitrogen fixation. In heterocysts the free nitrogen (N2) of the air is converted into (NO3)
  • Reclamation of alkaline “usar soils” can be done by employing some species of
  • Nostoc use as vegetable in China and


Important Tips

  • Cyanobacteria form a group of ancient gram negative photosynthetic
  • Nostoc colonies are found into the thallus of Anthoceros.
  • Heterocyst is the special cell in cyanobacteria which can fix to N2 from
  • Some blue green algae live in protozoans are called “cyanellae”.
  • The blue-green colour of cells is due to the presence of phycocyanin
  • Blue green algae were first placed under algae but now they are kept under bacteria
  • BGA produce oxygen during photosynthesis so these are called “oxyphotobacteria”.
  • A filament without mucilaginous sheath is called
  • In cyanobacteria, flagella are absent, PS-I and PS-II are present, photosynthesis is
  • Thick walled hormogonia or multicellular akinite found in blue green algae are called
  • Symbiotic N2 fixing algae is
  • Trichodesmium a non-heterocystous nitrogen fixing colonial aerobic cyanobacterium which occurs as phytoplankton throughout tropical and subtropical oceans and fixes atmospheric nitrogen while evolving photosynthetic



The term ‘virus’ has been derived from Latin, which means poison or venom or viscous fluid. These are highly controversial group of microscopic objects (smaller than bacteria, mycoplasma, nostoc, etc.) and are most perfect obligate intracellular parasites of the world. They remain inactive outside a living host but become active inside the host and multiply in it. They represent a transitional form of life between non–living and living world. Nowadays, these are defined as “Viruses are infectious nucleoproteins”. The definition given by Green (1935) states that the viruses are the smallest units showing reproductive properties considered typical of life.

According to Bawden (1949), “Viruses are obligate parasites, too small to be seen.”

Luria (1953) defined virus as “Sub-microscopic entities capable of being introduced into specific living cells and reproducing inside such cells only. “Single virus is called ‘Virion’, most of the plant virus are RNA virus. Must of the animal virus are DNA virus.

(1)  Important discovery of virus

  • Carolous causius (1576) recorded first viral disease in





  • Mayer (1886) found a disease in tobacco caused by virus and called it tobacco mosaic disease.
  • Ivanowski (1892), a Russian Botanist, discovered the infectious nature of the viruses. He was the person, who discovered the virus. He found that juice of an infected tobacco when filtered through bacteria–proof filter, caused disease in healthy plants of tobacco.
  • Beijerinck (1898) repeated the same experiment and called it infectious fluid–“contagium vivum fluidum”, therefore, he first used the word
  • Popper (1908) reported poliomyelitis
  • Twort (1915) and D. Herelle (1917) discovered bacteriophages, a kind of virus which infected bacteria and destroyed them.
  • Schelsinger (1933) for the first time isolated a virus by using the technique of ultracentrifugation
  • M. Stanley (1935) first time isolated tobacco mosaic virus (TMV) in crystalline form and showed that crystals were made up of proteins. Nobel prize was awarded to him for this work.
  • Bawden and Pirie (1938) purified TMV and found it to be a nucleoprotein containing
  • Saffermann and Morris (1963) discovered Cyanophages that infect blue–green


  • Nature of viruses : Viruses are regarded as intermediate between non-living entities and living organisms. It is very difficult to ascertain whether they are living or non-living. Some characters of viruses suggest their non-living nature whereas many other characters suggest their living The two views are listed below –






Fig : Influenze virus



Lipo protein envelope


  • Viruses are non-living : The following characters state that they are non-living.
  • Viruses have no complete cellular structure. They are not surrounded by cell membrane or cell wall.
  • They do not show cellular metabolism and lack respiration.
  • They possess high specific gravity unlike living
  • Viruses are active only when they are inside the living host Out side the host, they are good as chemical substances. Thus, they do not have their independent existance.
  • Stanley (1935) isolated the viruses in a crystalline form and kept for a long In this form they neither grow nor reproduce but remain in a crystalline form. This phenomenon has not been observed in any living organism.
  • The viruses can be precipitated just like chemical
  • Postulates of Robert Koch are not true for the Virus cannot grow in “invitro” condition in lab.
  • Viruses are living organisms : The following characters state that they are living organisms –
  • They have definite shape and morphology like that of a living
  • They possess genetic material (DNA or RNA), which determine their structure and Genetic material passes from generation to generation in usual manner.
  • All viruses are intracellular obligate parasite and attack specific hosts. The bacteriophages recognise the real bacterial The viruses produce characteristic symptoms on their particular host.
  • They show property of




  • They show irritability and respond to environmental conditions such as heat, ultraviolet rays, humidity, drought, alcohol,
  • They can grow inside the host and multiply enormously showing one of the most important property of living
    • Chemical composition : Chemically viruses are nucleoproteins. They are made up of central core of nucleic Nucleic acid is only one, either DNA or RNA. This nucleic acid (DNA or RNA) represents the genetic characters of virus. TMV has RNA (like most plant viruses have) 10% RNA and 90% protein is present in influenza virus and PSTV (Potato Spindle Tuber Viroid) also has RNA but it does not have capsid (protein coat). Plant viruses contain RNA but in cauliflower mosaic virus contain DNA. Bacteriophages contain DNA and almost half animal viruses contain RNA and half contain DNA. But it is called that often animal viruses contain DNA. Cancer causing viruses reovirus contain both RNA and DNA, protein is not genetic material and is left outside the host cell and nucleic acid enters into the cell during the process of infection virus can performs division only inside the host cell. Therefore, their metabolism is not independent. Some animals viruses may be coverd by a lipo-proteinaceous envelope Such viruses are called as Lipovirus. Influenza virus also contains carbohydrates. The envelope is made up of virus protein and host cell lipids. One specific feature of the envelope is that it is covered with projections called spikes. Only some enzymes are detected in viruses such as – Lysozyme in bacteriophages, transcriptase in vaccinia virus, reverse transcriptase and DNA or RNA polymerase in retroviruses.
    • Shape : There is variation in shapes of viruses. Viruses are always found in geometrical shapes. The virion may be spherical, oval, rod–like, brick–shaped or tadpole–like in On the basis of shape viruses have been placed in the following categories.
      • Straight, rigid rods with helical architecture, g. TMV, Barley stripe mosaic virus (BSMV).
      • Long flexous thread–like rods, g. Potato latent mosaic, Wheat streak mosaic virus.
      • Polyhedral virions, g. Turnip yellow mosaic, Tobacco ring spot virus.
      • Tadpole like – Bacteriophages.
      • Spherical – Influenza virus.
    • Size : Viruses have a long range of size. They range from 10 mm to more than 300 mm in size. The virus of foot and mouth disease (FMD) of animals is smaller than the largest protein molecule. The size of some viruses is as follows–Alfalfa mosaic virus is about 17 mm. Turnip yellow mosaic virus 20–30 mm, Maize stunt virus 240´50mm and Hydrangia spot virus is 44 ´ 16 mm. Smallest virus is foot and mouth disease virus (10 mm). Largest virus is smallpox virus – variola (250 mm).

(6)  General structure of virus

Structurally viruses are made up of envelope, capsid, nucleoid and occasionally one or two enzymes.

  • Envelope : Some viruses possess an outer thin loose covering, called envelope. It is composed of proteins (from virus), lipids and carbohydrates (both from host). The smaller subunits of envelope are called peplomers. Envelope is mainly found in some animal viruses (g., Herpes Virus, HIV, Influenza virus, Rous sarcoma virus) and rarely in some plant viruses (e.g., Potato yellow dwarf virus) and bacterial viruses (e.g., Pseudomonas Z). The viruses, which do not possess envelope, are called naked.
  • Capsid : It is the protein coat that surrounds the central portion of nucleoid and enzymes (if present). The capsid consists of a specific number and arrangement of small sub-units called capsomeres. These sub-units possess antigenic





  • Nucleoid : The nucleic acid present in the virus is called nucleoid. It is the infective part of virus which utilizes the metabolic machinery of the host cell for synthesis and assembly of viral components. The genetic material of viruses are of four types :
  • Double stranded DNA (ds DNA) : Occur in Herpes virus, Pox virus, Cauliflower mosaic virus (linear), Hepatitis–B virus (circular).


  • Single stranded DNA (ss DNA) : Occur in Coliphage fd (linear), coliphage
  • Double stranded RNA (ds RNA) : Occur in Reo virus, wound tumour

f ´ 174



  • Single stranded RNA (ss RNA) : Occur in Tobacco mosaic virus, Influenza virus, Foot and Mouth virus, Polio virus, Retroviruses (g. HIV), etc.


Tobacco mosaic virus (TMV) : It was discovered by the Russian worker D. Ivanowski. Franklin etal (1957) described the ultrastructure of (TMV) – It is a rod–shaped virus having a central core of RNA surrounded by protein coat (capsid) to form the nucleocapsid. The nucleocapsid may be naked or may be surrounded by a loose membranous envelope. It is composed of a number of subunits called capsomeres. The protein coat (capsid) consists of 2130 identical subunits (capsomeres). The protein is 94% and RNA is only 6%. In the entire length a single RNA molecule runs in the form of spiral coils. The molecular weight of RNA molecule is about 2 million. It provides a code which

Protein capsomere

RNA Coil


controls the amino acid sequence in the capsid. This virus measuring 300 × 18 nm.

Fig : The tobacco mosaic virus



Cryptogram of TMV-

R    E    S

:   :    :



1   5   E     *

1st pair : Type of nucleic acid / Number of strands in nucleic acid.

2nd pair : Molecular weight of nucleic acid in million / Percentage of nucleic acid in virus.

3rd pair : Shape of virus / Shape of capsid.

4th pair : Type of host infection / Type of vector

Bacteriophage : The viruses which attack bacteria are called bacteriophages. In outline they look like tadpole or sperm. The body can be divided into a hexagonal head


neck and a tail. The hexagonal head has a central core of DNA, which is surrounded by protein coat. The DNA is double helix, coiled molecule, about 50 mm in length. It is different from cellular DNA because it has hydroxymethyl cytosine (HMC) in place of cytosine. The cylindrical tail is hollow and is entirely made up of proteins. At the end of this, there are six long threads called tail fibres or caudal fibres. These fibres help the virus while attaching to bacteria. Bacteriophage contain lysozyme enzyme. The water of holy Ganga river contains bateriophage therefore bacteria cannot

grow in the water of Ganga.

650 Å













End plate

Head (DNA covered with protein coat)



Neck Tail

Tail fibres (6)


(about 200Å thick)

Fig : Structure of bacteriophage


Cyanophages : Generally some of the viruses are found which attack on blue green algae. Sofferman and

Morris (1963) reported 11 filamentous forms of blue green algae (Lyngbya, plactonema and phormidium, hence




called LPP-1) which were attacked by viruses. These viruses are usually called cyanophages. Cyanophages contain DNA as their genetic material. These viruses resemble with bacteriophages in morphology and behaviour. The cyanophages which acttack Nostoc (called N-1) and Anabaena variabilis (called An-1) are tadpole like whereas those which attack oscillatoria are rodshaped.

Mycophages : Some fungi such as, Mushrooms, Penicillium, etc have also been found to be infected by viruses. These are isometric in shape and contain double stranded RNA.

Phycophages : These are virus which attack on Algae.

  • Life cycle : The word reproduction is not appropriate in case of viruses because they have no cellular components or cell They do not reproduce themselves but divide by a special mechanism as follows.
  • Attachment : The bacteriophage gets attached to bacterial cell wall with the help of caudal
  • Penetration : Bacteriophage dissolves the bacterial wall by an enzyme Lysozyme and makes a pore in cell Through this pore DNA molecule enters in the cell after contraction of head protein, entire protein coat remains outside.
  • Latent period : Phage DNA controls hosts cellular Instead of formation of bacterial protein, phage protein formation begins. Cellular DNA and RNA is broken down and from this cellular DNA, phage DNA is formed. Now protein covers the DNA fragments to form a kid virus.
  • Maturation : This young virion is changed into an adult virus hence this process is called maturation. Head tail and tail fibres are formed independently in the cytoplasm and join in the main
  • Release : The viruses are mature, cell wall of bacterial cell is weakened by enzyme The release of viruses takes place by bursting of host cell and these are again ready for next infection or attack on other bacteria.
    • Transmission : It takes place by following means
      • By vegetative parts : This is the chief method of transmission of viruses in case of potato, raspberry, strawberry and other fruit
      • By seed : This method is found in few cases only like legumes, wild cucumber and
      • By mechanical means : Sometimes direct contact or mechanical media also transmits them. It may be by direct contact of healthy leaf to infectious leaf, rubbing infected juice on a healthy
      • By soil : Soil is a good media of transmission in case of potato mosaic virus, wheat mosaic virus, etc.
      • Insect transmission : Some insects like Aphids and leaf hoppers play role of vector similar to pollen transmission, g. leaf curl top.
      • By fungi : Viruses like Tobacco mosaic virus and Lettuce infecting virus are transmitted by fungi.
    • Symptoms of viral disease : The main symptoms
      • Mosaic spotting : Leaves show circular or irregular patches of white, light green or yellow colour, g.,

tobacco mosaic.

  • Ring spotting : This appears in localised spots in the form of concentric
  • Chlorosis : It shows uniformly disappearing of
  • Distortion : It is a common symptom showing rolling and curling of




  • Necrosis : In this symptom the host cells The symptoms may appears in various forms; it may be as patches on apical bud, on leaves or on stem.

(10)  Economic importance of viruses

Uses of viruses

  • Specific viral strains are cultured and attenuated to be used as vaccines against specific
  • The addition of cyanophages LPP-1 and SM-1 are useful in controlling water
  • Bacteriophage was used by Hershey and Chase to prove that DNA is the chemical basis of
  • Bacteriophages are of interest to geneticists because these bring about
  • Water of river Ganga is believed to have phages which destroy That is why its water does not get spoiled.

Viral diseases (Related to blood and organs)

  • Rabies or Hydrophobia (Highest mortality rate)
  • This virus is having single stranded
  • The shape or appearance of virus is bullet
  • AIDS : (Acquired Immuno Deficiency syndrome).
  • First case of AIDS was reported in Atlanta (1981).
  • This is suspected to be a monkey’s
  • 5-10 million people in the world are infected with this virus and in America alone 80%
  • Males are more susceptible to this disease than (92.5% in males, 6.5% in women and about 1% in children).
  • This virus spreads through blood transfusion, sexual contact,
  • This AIDS virus is known by different names as :

ARV : AIDS associated Retrovirus.

LAV : Lymphoadenopathy Associated Virus.

HTLV- III : Human T-cell Lymphotropic Virus Type –III.

HIV : Human Immunodeficiency virus.

  • This virus contains single stranded
  • AIDS virus likes T-lymphocytes which provide resistance to the organism through production of antibodies. This virus infects and kills T-lymphocytes (T-helper cells) and hence resistance of host is Thus man is infected with different types of infections. This is also known as Death Warrant.
  • Yellow fever : Transmitted by Aedes aegypti
  • Dengue fever : Transmitted by Aedes aegypti and Culex fatigans
  • Polio : Transmitted through food, water and contact, in five years below children. The polio virus is small, about 30 mm in dimeter and contains about 75% protein and 25% RNA polio and common colds are caused by picornaviruses having single stranded
  • Hepatitis A : Transmitted through food, water and
  • Hepatitis B : Transmitted through contact and body




  • Viroids : Diener and Raymer (1967) discovered very simple smallest infectious agents called Viroids consist of RNA only and capsid is lacking. Viroids contain only very low mol. weight. Diener and Raymer reported that causal agent of potato spindle tuber disease was a free RNA and no viral nucleoprotein particles were present in the infected tissue. T.O. Diener (1971) termed it viroid. Viroids are single–stranded, covalently closed circular as well as linear RNA molecules that occur in the form of collapsed circles and hairpin structures. Viroid RNA molecules are so small that the largest one so far described (CEV – citrus exocortis viroid) is only 371 nucleotides long–about one tenth of the size of the smallest RNA virus. Transmission is mechanical. The symptoms on host plants are almost similar to those of viruses. Viroids cause persistent infections. A number of other diseases caused by viroids are – Cadang Cadang of coconut, Cucumber pale fruit, Chrysanthemum stunt, Avacado sunblotch, etc.
  • Prions : Prusiner (1982) discovered it as a human disease causal agents. Stanley B. Prusiner discovered infectious agents which were Prions are proteinaceous particles thought to cause a number of diseases including the slow virus diseases, therefore also called as slow viruses. They are made of proteins molecules only. Genetic material (DNA and RNA) is absent in prions. These can multiply themselves and are infectious also. Prions can survive in heat, radiation and chemical treatment that normally inactivate viruses. Prions affect the central nervous system and one of the best known is the scrapie agent of sheep and goat which causes the animal to scrap itself against some objects. Kuru, a disease of central nervous system found in few canniblastic tribes of New Guinea is caused by prions. Other such disease is Creutzfeld–Jacob disease of humans and animals, similar to scrapie, gerstmann – strassler – scheinker syndrome. These all are diseases of central nervous system.
  • Interferons : M. Findley and McCallum (1937) reported a phenomenon called viral interference in which the cell infected with one type of virus becomes resistant to superinfection by other viruses. Alliac Issacs and Lindeman (1957) gave the term interferons to the chemical substances responsible for viral interference.
  • Interferons are produced by cells in mammals, rodents, birds, , and provide resistance against viruses.
  • Hilleman and Tydall (1963) isolated interferons from hen’s egg infected with influenza virus.
  • Interferons are protein molecules or polypeptides of low molecular weight which prevent viral

(14)  Some important- Aspects of virus

  • Plaque : Bacterial viruses are easily isolated and cultivated in young actively growing cultures of bacteria in broth or on agar plates, In liquid cultures, lysing of the bacteria may cause a cloudy culture to become clear, whereas in agar plate cultures, clear zones, or plaques, become visible to the unaided eye
  • The origin of Hela cells : It was in the winter of 1951 when Henrietta Lacks, a young black woman of 31, went to the medical clinic of Johns Hopkins University in Baltimore to seek medical The examining physician found a malignant tumor within her cervix. Some of this cancerous tissue was taken to a laboratory for cultivation.

In spite of intensive radiation treatment, the tumor continued to grow. Eight months after her first visit to the clinic, the cancer had spread throughout her whole body and she died. But the tumor cells taken form Henrietta Lacks thrived they divided and doubled their number every 24 hours. Cells taken previously from the tumors of dozens of other patients had not grown at all, or grew only poorly and then died off.

The cancer cells of Henrietta Lacks continued to flourish in culture in petridishes. These cells, now code named Hela cells, became one of the best known continuous tissue culture cell lines. Hela cells are widely used in research because they are so readily available, so versatile and so easy to propagate serially. They have been double the “cells that would not die.” Thus Henrietta Lacks left behind her the first widely available model of human tissue in vitro for scientific investigation. Perhaps her legacy will help to conquer the disease that vanquished her in 1952.

(iii) Plant viruses do not invade apical meristems

  • Apical meristems of even virus-infected plants are free from Majority of systemic viruses are not able to attack apical meristems.
  • Length of apices remaining free from the virus varies in different virus- host combinations as





100-200 mm for potato viruses, 400-1000 mm for sweet potato internal cork virus.

  • By excising and culturing the virus-free apical meristem, it is possible to prepare disease-free

Type of nucleic acid and number of strands in different viruses


DNA VirusesStrandsRNA VirusesStrands
AdenovirusesDNA (2)Avian leukemia virusRNA (1)
Bacteriophage f X 174DNA (1)Bacterial virus F2RNA (1)
Bacteriophage M13DNA (1)Bacteriophage MS–2RNA (1)
Coliphage lambda (l)DNA (2)Coliphage R17RNA (1)
Coliphage T2, T4, T6DNA (2)Influenza virusRNA (1)
Coliphage T3, T7DNA (2)Poliomylitis virusRNA (1)
Pox virusDNA (2)Tobacco mosaic virus (TMV)RNA (1)
Herpes virusesDNA (2)ReovirusRNA (2)
Popilloma virusDNA (2)Rice dwarf virusRNA (2)
Polyoma virus SV 40DNA (2)Wound Tumour virusRNA (2)

Families of animal viruses, grouped by type of nucleic acid


FamilyVirion StructureDiameter (nm)Examples/ Diseases
Papova virusNaked polyhedral40–57Papilloma (human warts, cervical cancer); polyoma (tumors in certain animals).
Adeno virusNaked polyhedral70 – 80Viruses that cause respiratory disease; some that cause tumors in certain animals.
Herpes virusEnveloped polyhedral150–250Herpes simplex I (cold sores); herpes simplex II (genital); varicella zoster (chicken pox, shingles); Epstein–Barr virus (infectious mononucleosis, Burkitt’s lymphoma).
Pox virusEnveloped complex200–350Variola (smallpox); vaccinia; cowpox.
ss DNA   
Parvo-virusNaked polyhedral18–26Most depended on coinfection with adenoviruses for growth
ss RNA that can serve as mRNA (+ strand RNA)
Picorna virusNaked polyhedral18–38Poliovirus; rhinovirus (common cold); enteric viruses
Toga virusEnveloped polyhedral40–60Rubella virus; yellow fever virus; encephalitis virus (transmitted by insects).
RetrovirusEnveloped polyhedral; two copies of genome per virion.100–120RNA tumor viruses (solid tumors and leukemia); AIDS
ss RNA that is a template for mRNA (– strand RNA)
RhabdovirusEnveloped helical70–180Rabies
ParamyxovirusEnveloped helical150–300Measles, mumps
OrthomyxovirusEnveloped helical; RNA in eight segments.80–200Influenza viruses
ds RNA   
ReovirusNaked polyhedral; RNA in ten segments.60–80Diarrhoea viruses
*ds = double– stranded; ss = single–stranded.





Important plant diseases caused by viruses


S.No.DiseaseCausal organism
(1)Abutilon mosaicAbutilon mosaic virus
(2)Bunchy top of bananaBanana bunchy top virus
(3)Cucumber mosaicCucumber mosaic virus
(4)Little leaf of brinjalBrinjal little leaf virus
(5)Little leaf of cottonCotton little leaf virus
(6)Papaya mosaicPapaya mosaic virus
(7)Potato leaf rollPotato leaf roll virus
(8)Potato mild mosaicPotato virus X
(9)Potato rugose mosaicPotato virus X and Y
(10)Stunt of S. C.Ratoon stunt virus
(11)Rosette of groundnutGroundnut mosaic virus
(12)Sugarcane mosaicSugarcane virus I
(13)Tobacco mosaicTobacco mosaic virus
(14)Tomato leaf curlTomato curl virus
(15)Tristeza of citrusCitrus Tristeza virus


Important human diseases caused by viruses


S.No.DiseaseHostCausal organism
(1)EncephalitisManEncephalitis virus
(2)Infectious hepatitisManHepatitis virus
(3)Herpetic KeratitisManHerpes virus
(4)InfluenzaManInfluenza virus–a
(5)MeaslesManMeasles virus
(6)Viral bronchitisManParainfluenza virus
(7)PoliomyelitisMan (children)Polio virus
(8)Small PoxManPox virus
(9)Common coldManRhino virus
(10)Yellow feverManYellow fever virus

Important Tips

  • Ivanowski (1892) a Russian botanist discovered virus.
  • Father of Virology M. Stanley (American Microbiologist).
  • Professor S. Bhargava is the specialist of virology in india.
  • (1892) studied canine rabies and used the term virus for the first time.




  • Edward Jenner (1796) developed the first successful vaccine against viral disease small
  • D’ Herelle (1917) coined the term “bacteriophage” for bacterial
  • Stanley (1985) an American biochemist, isolated and crystalized M.V.He was awarded Nobel Prize.
  • Caulimo virus (cauliflower mosaic virus) are double stranded DNA
  • Franklin etal (1957) described the ultrastructure of M.V.
  • Lindemann (1957) did the first successful vaccination against
  • Virus are made up genetic material and Capsid is made of protein. Unit of capsid is called capsomeres.
  • Single virus observed under electron microscope, outside host is called “Virion”.
  • The first virus to be cultured in human cells was Polio
  • Most of the phase are DNA
  • Mostly plant viruses have RNA and animal viruses have DNA as genetic
  • Single stranded RNA is found in M.V. and polio viruses.
  • Retroviruses have single stranded
  • Retroviruses and reverse transcription were reported by Temin and
  • Bacteriophage have single stranded
  • Viruses can pass through bacteria proof These are the intermediate connection between living and non living.
  • Viruses can not grow (multiply) out side a host They can grow in living cell only.
  • Some animal viruses covered by a lipo-proteinacous It also contains carbohydrate (found in influenza virus).
  • Viruses have host A specific virus infects only a particular host.
  • Polio vaccine was discovered by Salk and Sabine in
  • Virus are lack
  • In the world which do not have cell are virus, viroids and
  • Substance which can inactivate to viral activities are known as antiviral agents or
  • The synthesis of viral proteins takes place on host
  • Viruses lack pigments metabolic activity, they made up by RNA only movement and sex organs, but some enzyms are found in
  • Viroids : They are the smallest known disease causing agents in
  • Prions : They are causal agent of human disease and have no nucleic acid, only made up by protein
  • AIDS is caused by It infects T–lymphocytes. HIV virus remains dormant for about 8 years. Infected person does not suffer a symptoms during this period. AIDS day is 1st December.
  • Size of virus is – 300 nm. Largest virus is – vaccinia or cow pox-virus (500nm). Smallest virus is – Alfa-alfa virus (17 nm).
  • Smallest virus is Satellite virus or tobacco necrosis virus – 17
  • Viral diseases – yellow fever, influenza, small pox, polio, mumps
  • Pox virus is also known as vip
  • Five genes are present in a simplest


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