Introduction.                                                                                                                                                       

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.

 Bacteria.                                                                                                                                                               

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.

1. Occurrence and Distribution : The bacteria constitute a highly specialised group of one celled plants. 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 78^oC while in psychrophilic bacteria occurs upto the temperature of – 190^oC. The features which contribute to their universal distribution are –

1. Extremely simple structure.
2. Small size and consequent large surface–to–volume ratio. In order to maintain their small size, cell division occurs rapidly.
3. Resistance of vegetative cells to adverse environmental factors. Such as U.V. light desication. etc.
4. Formation of highly resistant endospores.
5. Diversity in their modes of nutrition.

2. Plant characteristics : The bacteria are microorganisms that possess rigid cell wall and when motile have flagella. They are unicellular organisms lacking true nucleus and membrane bounded cell organelles. The plant characteristics of bacteria are –
   1. Presence of a definite and rigid cell wall which in a few species contains cellulose.
   2. The tendency of some to grow as filaments.
   3. The ability of autotrophic bacteria to synthesize organic food from inorganic materials such as CO₂ and water.
   4. Structure of the bacterium cell and reproductive methods are similar to that of certain algae.
   5. Ability to synthesize amino acids from inorganic nitrogen.

 

 

 

3. Size : Bacteria are the smallest of all known cellular organisms which are visible only with the aid of microscope. 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.
4. 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.

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

 

Micrococcus

 

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

 

 

 

 

 

 

Vibrio

Mycelial

 

Spirillum

 

Streptobacillus

 

 

 

 

 

 

 

 

Stalked          Budded

or 64 as in Sarcina.

Fig Different forms of bacteria

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

Beggiota, Thiothrix etc.

6. Stalked : The body of bacterium posses a stalk e.g. Caulobacter.
7. Budded : The body of bacterium is swollen at places e.g. Retrodomicrobiom.

5. Flagellation : Depending upon the

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

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

A                B                C                        D                           E

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

2. Monotrichous (C) Lophotrichous (D) Amphitrichous (E)  Peritrichous

 

 

 

Lactobacillus

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

(6)Staining of bacteria

1. 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 dry. 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.
2. Gram staining : This technique was introduced by Hans Christian Gram in 1884. 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

 

 

 

 

 

 

 

 

 

 

Gram-Negative

Plasma Membrane

 

Periplasm

 

 

Mesosome

   Mesosome   Peptidoglycan

 

Lipopolysaccharide

Membrane

Teichoic acid + lipoteichoic acid

Gram-Positive

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 - Positive	Gram - 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 peptide	Diamino palmilic acid present in peptide.
(14)	O-antigen absent.	O-antigen present.

 

(7)Structure of bacterial cell

1. Capsule : In a large number of bacteria, a slimy capsule is present outside the cell wall. 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

1. It provides protection against phagocytosis and antibiotics.
2. Capsule also protects the cell against dessication and viral attack.

Type of capsule

1. Homopolysaccharide : When capsule are made by one type sugar e.g. Streptococcus mutans.
2. Heteropolysaccharide : When capsule are made by many type sugar e.g. Streptococcus pneumonae.
   2. Cell wall : All bacterial cells are covered by a strong, rigid cell wall. Therefore, they are classified under plants. Inner to the capsule cell wall is present. It is made up of polysaccharides, proteins and lipids.

1. In the cell wall of bacteria there are two important sugar derivatives are found i.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.

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

 

 

  Capsule  

 

 Fimbriae

 

 

 

  Polysome

 

Cell wall

Plasma membrane

Respiratory

chain      Cytoplasm

   Storage

   Granule

tRNA

 

 

Flagellum

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

mRNA

 Mesosome

Free enzyme

 

 

4. 

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.

5. The cell wall of Gram positive bacteria is much thicker and contains less lipids as compared to that of Gram +ve bacteria.
   3. Plasma Membrane : Each bacterial cell has plasma membrane situated just internal to the cell wall. 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 fission.
• It help in secretion and synthesis of material for cell wall.
• It receive DNA during conjugation and DNA replication enzyme.
• Mesosome participate in the formation of septa during cell division.
• 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 acids. 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.

1. 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 polyphosphate. 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.
2. 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 blue. By electron microscopy they appear as clear round areas.
3. Glycogen and sulphur granules : Glycogen are also known as polysaccharide granules. It can be stained brown with Iodine. 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.

5. 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. 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.
6. Plasmid : In addition to the normal DNA chromosomes many bacteria (e.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.

1. F-factor or fertility factor or F-plasmid : Which is responsible for transfer of genetic material from donar to recepient bacteria.
2. 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 antibiotic.
3. Colicinogenic factor : Which produces 'colicines' which kill other bacteria (other than which produce these colicines).

8. 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 organs. 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.

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

 

 

 

 

 

 

 

 

 

 

L ring

 

 

 

M ring

Filament

 

 

 

 

Hook

 

 

 

 

 

 

 

 

 

 

 

 

Peptidoglycan loyer

Outer membrane Peptidoglycan layer

 

Cytoplasmic membrane

2. 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.

 

3. Filament : It represents cylindrical hollow structure made up of protein monomers.

 

9. Pili or Fimbriae : Besides flagella, some tiny or small hair-like outgrowths are present on bacterial cell surface. 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.

Functions

 

1. The function of pili is not in motility but they help in the attachment of the bacterial cells.

 

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

 

10. 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.

 

 

 

11. Reproduction in bacteria : Methods of reproduction are following.

Vegetative reproduction

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

Time,h

Fig : Growth curve of bacteria

 

 

 

 

 

Cell wall                                 Cell membrane

 

DNA                        Cytoplasm

A

Parent cell

 

 

 

E                                                         B

2. Binary fission : It is the most common type of reproduction in bacteria during favourable conditions. 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

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

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

1. Endospore formation is more common in rod- shaped bacterial or bacillus forms. Position of single endospore may be terminal or sub-terminal or intercalary.
2. Endospore is having a characteristic structure, i.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

Exosporium

Spore coat

Cortax

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

3. Endospore is highly resistant to very high and very low temperature, strong chemicals and acids, etc., due to calcium dipicolinic acid and peptidoglycan in cortex. Dipicolinic acid (DPA) helps in stabilizing its proteins. DPA and Ca ions provide resistance to heat.
4. When favourable conditions come, outer layers rupture and active bacterial cell comes out. So this is a method of perennation (i.e., to tide over unfavourable condition) and some people say it “ reproduction wihtout multiplication”.
5. The bacterial spore or endospore is perhaps the most resistant living structure known to science.
6. Tetanus causing and anthrax causing bacteria produce endospores.

2. By conidia : These are found in filamentous bacteria like streptomyces. The conidia are spore like structure formed in chains. Each conidium gives rise to a new bacterium.
3. 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.

1. Transformation : In this process one kind of bacterium is transformed into another kind. It takes place by transferring DNA from one bacterium to another bacterium. 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.

1. Avirulent strain
2. Virulent strain

• I¾nje¾ct in¾m¾ice® Healthy mice.

• I¾nje¾ct in¾m¾ice® Mice die.

3. (Heat killed virulent strain) Bacterial strain  ¾¾^Inje¾^{ct in}¾^m¾^ice® Healthy mice.
4. Avirulent +(Heat killed virulent bacterial strain)  ¾¾^Inje¾^{ct in}¾^m¾^ice® 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.

Capsulated

 

 

 

 

Heat killing

Non Capsulated

 

 

 

 

 

 

 

 

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.

2. 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 (i.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

DNA

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.

1. Generalised (non-specific) transduction which can transfer any phage sized fragment of host DNA.
2. Specific transduction which is restricted to the transfer of specific portion of DNA.

3. 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 (E.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