Microbial growth is defined as the increase in number of cell in
a population. Growth Curve: graphical expression of microbial
growth. Lag, log, stationary and death phaseLag phase: When a
microbial culture is inoculated into a fresh medium, growth usually
begins only after a period of time called the lag phase. Log phase
: phase in which the cells double during a constant interval.
Stationary Phase: In the stationary phase, there is no net increase
or decrease in cell number and thus the growth rate of the
population is zero.Death Phase : phase in the growth when the cells
start dying and the graph curves downwards Lag Phase: Lag phase is
the result of change of conditions for microorganisms Duration of
lag phase depends on various parameters such as; temperature, pH,
nutrients and age of the culture Lag phase is the time taken to
adjust with the new growing conditions. Is it reduce the duration
of la phase ? Log phase is the phase in which bacteria show
exponential growth.During this phase generation doubles with
constant interval As a result the graph shows a straight line in a
semilogarithmic graphMetabolic activity will be on high rate in log
phase.This phase is used for the calculation of generation time
Rates of exponential growth vary greatly. The rate of exponential
growth is inuenced by environmental conditions (temperature,
composition of the culture medium), as well as by genetic
characteristics of the organism itself.
Stationary PhaseIn a batch culture exponential growth is
limited.After exponential growth essential nutrients is almost used
up and bacteria produce large amount of waste products This
conditions reduce growth rateBacterial metabolism reducesSome cells
divide and some die as a result growth rate become almost zeroThis
is a phenomenon called cryptic growthUsually during stationary
phase bacteria produce many secondary metabolites During this stage
bacteria frequently produce a variety of starvation proteins, which
make the cell much more resistant to damage in a variety of ways
They increase peptidoglycan cross-linking and cell wall
strengthChaperones prevent protein denaturation and renature
damaged proteins. As a result of these and many other mechanisms,
the starved cells become harder to kill and more resistant to
starvation itself, damaging temperature changes, oxidative and
osmotic damage, and toxic chemicals such as chlorine.
Death PhaseDetrimental environmental changes like nutrient
deprivation and the buildup of toxic wastes lead to the decline in
the number of viable cells characteristic of the death phase.Some
time the dead cell fail to lyse as result the optical density of
the medium can remain constant and the graph can be a straight
line.Measurement of Microbial growth:Direct countingThe
Petroff-Hausser Counting ChamberPlating methods TVCFiltration
method Measurement of cell massWet weightDry weight
Measurement of cell density Optical density by
spectrophotometer
