Reproduction in Bacteria
Category : 11th Class
Vegetative reproduction
(1) By budding : According to Bisset and Hale, reproduction by budding takes place in Bigidi bacterium bifidus.
(2) By binary fission : This type of reproduction is most common in all kinds of bacteria. Under favourable conditions bacterial cell expands. Cytoplasm divides into two parts due to constriction and formation of a transverse septum in the centre of the cell. Later on, these two parts separate from each other and give rise to two cells.
The speed of binary 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 endospores : Endospores are formed in all species of the genera Bacillus and Clostridium. In each cell only one endospore is formed. Endospore is highly resistant to very high and very low temperature.
Endospore is found either in the centre or near the cell wall. Under unfavourable conditions cytoplasm shrinks and gets rounded and around it a hard protective three layer is formed. Each endospore may be either circular, ellipsoidal or semicircular. 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”.
(2) By conidia : Some filamentous bacteria e.g., Streptomyces reproduce by means of conidia. The conidia are spore like in structure and are formed in chains. Each conidium gives rise to a new bacterium.
(3) By zoospores : In rare cases bacterial cell forms some motile spores which give rise to new cells. This process has been rarely seen. e.g., Rhizobium.
Sexual reproduction (Genetic recombination)
Sometimes it was believed that sexual reproduction does not take place in bacteria. Lederberg and Tatum (1946) proved that sexual reproduction takes place in bacteria. On the basis of this discovery they were awarded Nobel Prize.
According to the present view, three types of sexual reproduction are found in bacteria :
(1) Transformation : In this process one kind of bacterium is transformed into another kind. It takes place by transferring DNA from capsulated to non-capsulated bacterium. For the first time Griffith (1928) reported transformation in mice. Later on, Avery, Mcleod and McCarty (1944) studied transformation in Diplococcus pneumoniae.
(2) Transduction : In this process DNA of a bacterial cell is transferred into another bacterial cell through bacteriophage – a kind of virus which is parasitic upon bacteria. Bacteriophage consists of DNA. It has been now accepted that DNA of a bacterial cell is transferred through bacteriophage to another bacterium. Transduction was first of all reported by N.D. Zinder and Lederberg (1952) in bacteria Salmonella typhimurium.
(3) Conjugation : In this process genetic material from one strain of bacterium which is known as male is transferred into another strain of bacterium which is known as female. On the experimental basis it is believed that genetic material of male enters into female bacterium in the form of a straight line. Lederberg and Tatum first of all reported conjugation in bacterial strain of E.coli called K12 (1946). In 1966, Wollman and Jacob described it in details.
In gram negative bacteria, there are two strains, F+ (with fertility factor and sex pili) and F– (without fertility factor and sex pili). These two can come together. Sex pilus of donor cell extrudes a protein that helps it in attaching to the recipient cell. Latter on, sex pilus is converted into conjugation tube between the two. The donor or F+ can transfer its fertility factor or plasmid to recipient cell or F– and convert it into donor as well. Sometimes the F+ plasmid attaches to nucleoid, becomes episome and converts the donor into HFr (high frequency of recombination 1 : 100). There is a transfer of few genes of nucleoid from HFr to F–. HFr quality can also be transferred occasionally (HFr × F– = F– plus a few genes and a few HFr). The transferred segment is called exogenote which 'similar' segment of the recipient bacterium is known as endogenote. The recipient bacterium is called merozygote or partial zygote. Some of the donor genes integrate into recipient genome.
In gram positive bacteria sex pili do not develop. Donor cells produce a protein adhesion over their surface for bringing recipient cells in contact with them. In Streptococcus faecalis, the recipient cells excrete a small peptide (sex hormone) for inducing clumping. Wall dissolves in the region of contact. Transfer of DNA segment occurs from donor to recipient cell.
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