Reproduction

Asexual reproduction

Bacteria reproduce asexually by binary fission and spore formation.

Binary fission

—This is the universal method of multiplication in bacteria in which the cell divide into two equal parts by a transverse constriction of the cytoplasm.

—Then nucleus divides amitotically and the process of binary fission is completed in 20-30 minutes.

Fig. Binary fission in a bacterial cell

Spore formation

—Bacteria produce several types of spores called gonidia, sporangiospores, arthrospores (oidia), conidia, cysts and endospores. Process of sporulation occurs under unfavourable condition. Some rod shaped bacteria like Bacillus and Clostridium form spores inside the vegetative cell called endospores.

—Endospores are highly thick-walled and resistant spores which are formed in response to adverse environment or presence of harmful waste products.

—Cocci and spirilla bacteria do not produce endospores.

—This is mainly a source of perennation and dispersal and not a source of reproduction. It can easily tolerate a temperature of 100°C to –100°C. Only two of the pathogenic bacteria produce endospores which are tetanus and anthrax bacteria.

Sexual reproduction

Typical sexual reproduction is absent in bacteria because of absence of haploid-diploid alternation.

But gene recombination occurs by four methods - Conjugation, transformation, sexduction and transduction.

Conjugation

—It was first discovered in Escherichia coli by Lederberg and Tatum. Bacterial conjugations are dimorphic, i.e., they have two types of cell, male (F+) or donor and female (F–) or recipient cell through the conjugation tube.

—The original genome of the recipient cell is called endogenotes and the fragment of DNA transferred from the donor cell is called exogenote.

—Wollman and Jacob studied details of the conjugation.

—The conversion of F– bacterium into F+ following the infection by episome DNA provides another evidence that DNA is the genetic material.

Transformation

—It is the oldest process of genetic transfer, Griffith reported transformation for the first time in Pneumococcus pneumoniae. Avery, Macleod and McCarthy studied this process in details. Transformation is a laboratory procedure and rarely occurs in nature.

—During the process of transformation of genetic material between the bacterial cells, it is considered that a part of DNA of one bacterial cell gets adsorbed on the surface of the recipient cell and finally absorbed by the latter.

Sexduction

—It has been discovered by Jacob and Adelberg.

—In this process the Hfr males may revert back to the normal F+ males by normal separation of F-factors.

—During the separation of F-factor from bacterial genome, the F+-male may carry some of the bacterial genes due to faulty separation called sexduction.

—The resultant males are called F´ (Prime males).

Transduction

—This process was first studied by Zinder and Lederberg.

—It is the transfer of foreign genes by means of virus.

Economic importance of bacteria

Bacteria play significant role in day to day activities of human beings.

Beneficial activities

Role in Agriculture

—Decay and decomposition of organic matter, sewage disposal, nitrogen cycle, nitrogen fixation, gobar gas plants, etc.

Role in Industry

Man has utilized the metabolic activities of bacteria in preparation of a number of industrial products as described below:

—Butter milk and sour cream, yoghurt, cheese, vinegar, retting, leather industry, etc.

Role in Medicine

Bacteria and Actinomycetes have been used extensively in preparation of anitibiotics, vaccines, serums and vitamins. Maximum antibiotics are obtained from the group Actinomycetes.

Table : Antibiotics obtained from Actino,ycetes and Bacteria

Antibiotic Source

Streptomycin Streptomyces griseus

Terramycin or oxytetracycline S. ramosus

Erythromycin S. erythreus

Chloromycetin or chloramphenicol S. venezualae and S. lavendulae

Neomycin S. fradiae

Viomycin S. puniceus

Kanamycin S. kanamyceticus

Novoiocin S. niveus

Nystatin S. noursei

Polymixin Bacillus polymyxa

Thyrothricin B. brevis