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Reproduction in Bacteria
1. By Binary fission
It occurs normally under favorable conditions. The cell elongates and becomes
constricted at its middle. The cytoplasm divides and the constriction
increases until the cell is split into two cells ( Fig. 5). The generation time
(time needed for the cell to divide into two) differs according to species and
prevailing conditions. The generation time ranges from 20 minutes to 6
hours. In 24 hours , under optimum conditions a single individual would
produce several billions, a behavior which does not prevail due to the
interference of other factors.
Fig. 5. Binary fission in bacteria.
Binary fission
2. By Endospore formation.
They are small spherical or oval bodies formed within the cell, due to the
contraction of the cytoplasm. They become surrounded with a thick wall
and appear as glistening bodies which stain poorly.
They are capable of resisting unfavorable environmental conditions for
many years. They can withstand boiling, desiccation and high
concentrations of disinfectants. When conditions become favorable, the
spores germinate and are transformed into vegetative cells. Spore
formation usually occurs in rod-shaped bacteria (Bacillus and Clostridium).
The formed spore may be located either : a) centrally , b) terminally or
c) sub-terminally (Fig. 6). In some species the spore diameter is greater
than the width of the bacterial cell.
Central
endo-spore
Terminal
endo-spore
Fig. 6. Endospore formation
Sub-terminal
endo-spore
The sporulation cycle of Bacillus subtilis.
• The sporulation process is not one of multiplication , since
most rod-shaped forms produce only one spore each. It is
considered as a method for keeping the organism intact and
surviving during un-favourable conditions.
3. By Conidia formation
It occurs in members of certain families
belonging
to
Actionmycetes,
e.g.
Streptomyces. The ends of the filaments
become septated forming a chain of conidia
similar to those produced by true fungi. These
conidia can resist desiccation (similar to
endospores) but they cannot resist high
temperatures.