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Asexual reproduction in Bacteria

Asexual reproduction in bacteria takes place by following ways.

1.     Binary Fission

  • It is the simplest and most common way under favourable condition. In this bacterial cell simply divides into two by transverse wall (Fig 9).  The various events of binary fission are as follows:

  • The duplication of DNA and mesosome takes place and get separate from each other.

  • The daughter mesosomes and nucleoids migrate towards the opposite poles. Other cytoplasmic components are distributed to the two developing cells.

  • The septum (partition wall) at the centre begins to grow inward as the chromosomes move towards opposite ends of the cell.

  • Finally, the parent cell divides into two identical daughter cells.

  • Different from mitotic division as it does not involve spindle formation and does not ensure equal division of DNA.

  • Binary fission occurs very rapidly. Some species of bacteria can double their population in less than ten minutes e.g., E. coli (20 minutes).

 2.     Budding

  • Some bacteria multiply by budding and in this case the cell protoplast covered by the thin wall bulges out in the form of a small protuberance.

  • The protuberance gradually increases in size and is termed as bud (Fig 10).

  • Then the parent DNA divides into two portions. One of these migrates into the bud.

  • The bud then detached from the parent cell by constriction and grows into new cell.

  • It is comparatively rare process and observed in few bacteria like Rhodopseudomonas, Hyphomicrobium, Pedomicrobium, Hyphomonas etc.

  • Budding is different from binary fission as in this mother cell retain its identity even after formation of new cells.

 3.     Cysts

  • Cysts are formed in certain bacteria and in this the entire protoplast of the cell recedes from the cell wall and becomes rounded. e.g., Azobacter

  • A thick wall is then secreted around it to form resistant structure.

  • This extra thick wall around the existing cell wall protects it from harsh condition.

  • Under suitable environment conditions the cyst germinates to produce the new bacterium.

 4.     Segmentation

  • In some species of bacteria, the protoplast of the bacterium cell at some stage divides to form very tiny body called gonidia. 

  • The cell wall ruptured and the liberated tiny gonidia grow into new bacterium cell under suitable conditions.

 5.     Endospores

  • Endospores are highly thickened resting intracellular cell and was reported by J. Tyndall and are formed in both aerobic and anaerobic forms.

  • It develops within vegetative bacterial cells of several genera of gram-positive bacteria (Bacillus, Clostridium, Sporosarcina).

  • The endospore consists of central protoplast called core which mainly contain DNA, RNA, ribosomes, enzymes etc.

  • Core is covered by thin membrane called core wall or germ membrane or inner membrane (Fig 12).

  • Outside the core wall is multi layered region called cortex and it is very large and occupies half of the spore volume. Cortex mainly composed of calcium, dipicolinic acid and peptidoglycan.

  • Cortex is followed by spore coat (inner and outer coat) and mostly composed of proteins. It is comparatively thick and main protective agent.

  • Outer most layer is exosporium and it is thin made of polysaccharides, proteins and some lipids.

  • Endospores can tolerate very harsh condition, heat resistant even at 80o C for 10 minutes, tolerate drought and survive for years (200 years). It even withstands acid treatments (H2SO4) and are extremely hard to kill.

  • Because of their resistance several species of endospore forming bacteria are dangerous pathogens.

 

6.     Conidia

  • A conidium develops by the formation of a transverse wall at the tip of the filament in many filamentous bacteria e.g., Streptomyces. 

  • The filament bearing conidia are known as conidiophores.

  • After separation from parent bodies each conidium gives rise to a new filamentous bacterium.

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