REPRODUCTION OF BACTERIA - VEGETATIVE, ASEXUAL, CONJUGATION, TRANSDUCTION AND TRANSFORMATION

            Bacteria reproduce vegetatively, asexually and sexually. Bacteria can reproduce at tremendous speeds.  Some bacteria can reproduce as often as once every 20 minutes.  However, bacteria have to have certain conditions in which to reproduce. These conditions are not often met, and that is one thing that keeps bacteria from growing out of control.

            There are hundreds of thousands of bacterial species in existence on Earth. They grow relatively quickly, and most reproduce by binary fission, the production of two identical daughter cells from one mother cell. Therefore, each replication cycle doubles the number of cells in a bacterial population.

            Bacteria reproduce using three basic methods –  vegetative, asexual and sexual reproduction.

            A. VEGETATIVE REPRODUCTION:-  Vegetatively bacteria reproduces by the following methods –

            (a) Fission:- It is very common and simple method of reproduction in bacteria. In this process the single cell divides into two cells by a process called binary fission. After division each half, i.e., the daughter cells behave as independent individual cell. The daughter cells divide repeatedly to multiply in numbers. By this method a single bacterium may produce 16 million bacteria in 12 hours.

            (b) Budding:- It is also another type of vegetative reproduction found in bacteria. In this type one small outgrowth comes out from the bacterial cell. The outgrowth enlarges and on maturity, separates from the mother cell and behaves as an independent cell.

            (c) Segmentation:- Some other species of bacteria reproduce vegetatively by a process called segmentation. In this case, the protoplast of the bacterial cell, at some stage, divides to form very tiny bodies called gonidia. The cell wall ruptures and the liberated tiny gonidia grow into new bacterial cell under suitable conditions.

            B. ASEXUAL REPRODUCTION:- There are four of asexual reproduction in bacteria. These are – endospore formation, conidia formation, myxospores formation and cysts formation.

            (a) Endospore Formation:- Under unfavorable condition, endospores are formed. They are a special type of reproductive bodies which are produced by bacterial cells. At the time of endospore formation, the bacterial cell enlarges and loses its flagella. The protoplasm concentrates in the middle portion of the cell and develops a thick wall around it which is known as endospore. Sometimes two endospores are formed in the same cell. Examples – Bacillus, Clostridium, etc.

            (b) Conidia Formation:- Streptomyces, an important member of Actinomycetes produce conidia. Conidia are produced externally in chains on the aerial filament of fungus-like bacteria.

            (b) Myxospores Formation:- Myxospores are the spores of Myxococcus and other myxobacteria. These spores develop from fresh vegetative cells which are not divided of their genetic material.

            (d) Cysts Formation:- In some species of Azatobacter, the entire cell is transformed into a resistant structure called cyst. At the time of cyst formation, vegetative cell first becomes deflagellated, attains spherical shape and gets surrounded by multilayered coat.

              C. SEXUAL REPRODUCTION:- Bacteria do not reproduce sexually in the normal sense as eukaryotes as they have no gametes, no alteration of generation and no meiosis. Genetic Recombination, however, takes place in bacteria by these three mechanisms – Conjugation, Transduction and Transformation

            1. Conjugation:- During conjugation transfer of DNA takes place between bacterial cells. It requires cell-to-cell contact. One bacterium connects itself to another through a protein tube structure called pilus. Joshua Laderberg and Edward Tatum first described conjugation in 1946. They discovered the F^- factor, which can move between E. coli cells. F^- factor is a plasmid (plasmids are circular extra chromosomal DNA). These plasmids carry antibiotic resistant genes called R factors. These also allow movement of DNA from a donor to a recipient cell. This is the process of conjugation.

               Bacterial conjugation involves various steps like mating pair formations, conjugal DNA synthesis, DNA transfer and maturation. After a stable mating pair is formed DNA replication starts it produces a single stranded copy of the F^- factor DNA. This DNA strand is transferred to the recipient cell. Here it is copied to make a double stranded DNA molecule. This then forms a circular plasmid. Towards the end of conjugation, the mating pair is broken and both the donor and the recipient cell carry an identical episomal copy of F^- factor.

          2. Transduction:- This is the second way by which DNA is transferred between bacterial cells. It takes place thru a phage particle. Joshua Lederberg and Norton Zinder discovered transduction in 1956. Phages inject their DNA into a recipient cell and kill the host cell (lytic growth). Some phages do not always kill the host cell (temperate phage), these can be inherited by daughter host cells. This acquiring of a temperate prophage by a recipient cell is a form of transduction.

            Transduction is of two types: Generalized and Specialized.

            a. Generalized Transduction – During this process any bacterial gene can be transferred from a donor cell to a recipient cell. This maybe done via a bacteriophage and it carries only bacterial DNA and no viral DNA. This is packaging of bacterial DNA into a viral envelope. If the virus replicates it fills the nucleocapsid with genetic material.

            b. Specialized Transduction – It is the process by which a particular set of bacterial genes is transferred to another bacterium. The genes that get transferred are the donor genes. The bacteriophage  can pick up a subset of chromosomal genes and transfer them to other bacteria, e.g., bacteriophage such as phage lambda. Lambda can establish a lysogenic infection in a bacterial cell. In this infection the viral DNA gets incorporated into the host chromosome. When the conditions are appropriate the integrated viral DNA can cut itself from the chromosome and enter the lytic phase in which the virus replicates. The cell gets lysed and the new bacteriophage particles are released to infect other cells.

               3.     Transformation:- Bacteria also exchange DNA by transformation. Free DNA from outside of living bacterial cell, which is released from the dead and lysed bacterial cells is transported into the cytoplasm in some bacteria. Bacteria that undergo transfer of DNA by this method are called ‘naturally competent’. O. Avery et al. for the first time performed this type of DNA transformation of Streptococcus pneumoniae. E. coli  is an example of bacterial species which is not naturally competent for DNA transformation. The double stranded DNA gets bound to the cell surface of a recipient by a protein receptor. A strand of DNA gets transported through cell envelope. It can recombine with similar DNA sequence present in the recipient cell. This happens in natural DNA transformation.

************