The bacterial growth curve has four distinct phases viz. Lag phase, Log phase, Stationary phase, and Decline phase (Fig).
(a) Lag Phase: The bacteria first undergo a period of
acclimatization when they are inoculated into the sterile nutrient medium.
During that time, necessary enzymes and intermediate metabolites are
synthesized, and before multiplication, the bacterium reaches a critical stage
and finally, multiplication takes place at this stage. Hence, this stage is
also known as an initial stage or dormant stage. The duration of this phase
depends on the type of bacteria, quality of culture medium, size of inoculum,
and some environmental factors like CO2, temperature, pH, etc. The
average time of lag phase is 2 hours but it varies from species to species and
ranges between 1-4 hours.
(b) Log Phase: After the Lag phase the next phase is the
Log phase. In this phase, the bacteria undergo cell division and their
population increases exponentially at a logarithmic rate. Hence this phase is
also known as the exponential phase. The number of viable counts, when plotted
against time gives a straight line of inclined fashion. The average time of the
log phase is 8 hours but also varies with the bacterial species.
(c) Stationary Phase: In this phase, the cell
division of bacteria almost ceases due to the exhaustion of nutrients and also
the accumulation of toxic products. Secondary metabolites like antibiotics are
also produced in this phase. The cell death starts at a slow rate and is
compensated by the formation of new cells through cell division. The total cell
number increases at a slow rate, but the viable count remains almost constant.
The duration of this phase varies from a few days to a few hours.
(d) Decline Phase: This is the last stage of the bacterial
growth cycle. In the phase of decline, the total number of cells remains
constant, but the number of viable cells gradually decreases due to the exhaustion
of nutrients and also the accumulation of toxic products. In some cases, a few
cells remain viable for a long time. These viable cells probably grow by
utilizing nutrients released from dead cells. The cells attain maximum size at
the end of the lag phase and become smaller in the log phase.
