Bacteria mostly divide by binary fission.
When a bacterial cell reaches a certain size, it divides to form two daughter
cells. Nuclear division precedes cell division, and therefore, in a growing
population, many cells carrying two nuclear bodies can be seen.
The
interval of time required for a bacterium to give rise to two daughter cells
under optimum conditions, is known as generation time or population
doubling time. The generation time in vary in different types of
bacteria. In coliform bacilli the generation time is about 20 minutes. In
tubercle bacilli it is about 20 hours and in lepra bacilli, the generation time
is as long as 20 days.
Bacterial
growth may be considered at two levels – (a) increase in size of the individual
cell and (b) an increase in number of the cells in a population. Growth in
numbers can be studied by bacterial counts.
*Bacterial Growth Curve
If
bacterial counts are made at intervals after inoculation and plotted in
relation to time, a growth curve is obtained. The curve shows the following
phases –
1. Lag Phase:- Immediately after the
inoculation in a culture medium, there is no appreciable increase in number of
the cells. This initial period is the time required for adaptation to the new
environment, during which the necessary enzymes and metabolic intermediates are
built up in adequate quantities for multiplication to preceed.
2. Log Phase:- Following the lag phase,
the cells start dividing rapidly and their numbers increase exponentially or by
geometric progression with time. Here, the curve shows the maximum rise due to
the rapid increase in the number of cells in the population.
3. Stationary
Phase:- Following the log phase, cell division stops due to the depletion
of nutrients and accumulation of toxic products. The number of progeny produced
is just enough to replace the number of cells that die. The viable count
remains stationary as equilibrium exists between the dying cells and the newly
formed cells.
4.
Decline Phase:- It is the phase when the population decreases due to cell
death. Besides nutritional exhaustion and toxic accumulation, cell death may
also be caused by autolytic enzymes.
When the population growth is plotted against
time a curve is obtained which is as follows.
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