GROWTH CURVE OF BACTERIA
Bacteria are unicellular, prokaryotic, free living
microorganism most commonly
replicate by the asexual process of binary fission. When grown in
culture, a predictable pattern of growth in a bacterial population occurs.
This pattern can be graphically represented as
the number of living cells in a population over time and is known as a bacterial growth
curve.
Normal
growth curve of bacteria can be determined by inoculating a small number of
bacterial cells into a suitable culture medium and counting the bacteria in
aliquot samples at regular intervals.
When
the logarithms of the viable cells are plotted against time on a graph paper,
it gives a typical curve called as bacterial
growth curve or growth cycle of
bacteria.
The resulting bacterial growth curve has the
following four distinct phases:
- Lag phase
- Log or logarithmic or exponential phase
- Stationary phase
- Death or decline phase.
LAG
PHASE:-
This is the initial phase characterized by cellular activity but
not growth. When bacteria are inoculated into a fresh medium, the multiplication of bacteria
does not start immediately. the microbial population, remains
constant for an initial period. The period between inoculation and the
beginning of multiplication is known as the lag phase.
In this phase bacterial cells adjust
itself to adopt the new environment. The enzymes coenzymes and other essential
molecules are synthesized by the bacterial cell during this phase.
During this phase, there is an increase in size of bacteria but no
appreciable increase in number of bacterial cells. The
cells are metabolically and physiologically very active but do not divide.
The
length of the lag phase depends upon the nature of medium, species of
microorganisms and other various physical and chemical growth factors; it may vary from 1 hour to
several days.
LOG PHASE (EXPONENTIAL
PHASE):
After the initial
phase or lag phase, bacterial cells enter in to the log phase. During these
phase the cells are dividing steadily at a constant rate by binary fission and
doubling in numbers. The time required for one bacterial
division during this phase is known as generation time. the log of the number
of cells plotted against time results in a straight line. The bacteria multiply
at their maximum rate and their number increases exponentially or by geometric
progression with time. In these phase the metabolic activity is high
as DNA, RNA, cell wall components, and other substances
necessary for growth are generated for division. The number of
bacteria present in each generation period is almost twice that in the previous
period
All bacteria do not
have the same generation time. E.g. Escherichia
coli may have 15 10 20 minutes, staphylococcus
aureus, 25 to 30 minutes, mycobacterium
tuberculosis may have 780 to 940 minutes. Generation time is mainly
dependent on type of species, nutrients in the medium and physical conditions.
STATIONARY PHASE: -
After
log phase the stationary
phase started in this phase a constant high population of cells is maintained
by a balance between cell division and cell death. In these phase the rate of
multiplication is reduced due to lack of essential nutrients, lack of
water oxygen, change in pH of the medium, etc. and accumulation of their own
toxic metabolic wastes.
At the
cellular level during stationary phase reserved food materials get consumed, a
proportion of ribosome’s may be degraded and enzymes may still be synthesized.
A viable population count at this stage shows no change.
In this phase spore forming bacteria
produce endospores and pathogenic bacteria begin
to generate substances (virulence factors) that help them survive harsh
conditions and consequently cause disease.
DEATH OR DECLINE PHASE: -
Death phase are also known as the logarithmic
death phase. During death phase, the number of viable cells decreases
exponentially, and population growth experiences
a sharp decline.
Bacteria die at different rates, just as they
grow at different rates, between each of these phases, there is a small curved
portion called the transitional period.
The
death phase starts due to the variety of conditions but the most important are.
Ø Depletion of nutrients
Ø Accumulation of toxic waste products and
autolytic enzymes.
Involution and irregular forms are common in
this phase. Some cells assume various shapes, becoming long, filamentous rods
or branching or globular forms that are difficult to identify.
Some develop without a cell wall and are
referred to as protoplasts, spheroplasts, or L-phase variants (L-forms). When these involuted forms are inoculated into a fresh nutrient
medium, they usually revert to the original shape of the healthy bacteria.
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