Bacterial growth occurs in four distinct phases: 1. Lag phase: Bacteria adapt to their environment and produce enzymes needed for growth. No cell division occurs. 2. Log/exponential phase: Bacteria divide rapidly in a geometric progression. The population doubles at regular intervals. 3. Stationary phase: Growth slows as nutrients become scarce and waste builds up. The growth rate equals the death rate. 4. Death phase: As nutrients are depleted, the death rate exceeds the growth rate and the population declines.

1. Bacterial Growth Curve

2. Bacterial Growth curve
• During typical bacteria growth (growth cycle) , bacteria cell divide by binary
fission and their mass and number increase in an exponential manners.
• Growth - increase in cellular constitutes of an organisms.
• Population growth - increase in number of cells in a population.
• Cell growth - increase in size of an individual microbial cell.
• Growth in multicellular organisms - increase in size
• Growth in unicellular - increase in number of individuals in population.
• If the microbial concentration of a batch culture is periodically measured, then a
curve describing the change in cell number against time can be drawn. The
resulting curve called the growth curve, has 4 distinct phases.

3. Bacterial Growth curve

4. Stages of Bacterial Growth
• Lag Adapt to nutrients
• Log Active Growth
• Stationary Growth equals death
• Death Nutrients Consumed

5. Bacterial Growth curve- Lag Phase
1. Lag phase:
•When microorganisms are introduced into fresh culture
medium, usually no immediate increase in cell number
occurs. This period is called the lag phase.
•During this phase , the cells are not dormant , but they
show intense metabolic activity like transporting
nutrients inside the cell from medium , synthesizing
DNA and various enzymes required for cell division.

6. Bacterial Growth curve- Lag Phase
• This is period of intense physiologic adjustment involving the
induction of new enzymes and the synthesis and assembly of
ribosome.
• There would be increase in the size of cell without any multiplication
for same time.
• In lag phase and during this phase there occur:
1. Increase in size of cells
2. Increase in metabolic rate
3.Adaptation to new environment and necessary enzymes.
4. Increase in cell protein, DNA, dry weight
5. No change in number of cells

7. Bacterial Growth curve- Lag Phase
• The length of lag phase depend upon
a. Type of bacteria and status of bacteria ( normal / damaged)
b. Better the medium, shorter the lag phase.
c. The phase of culture from which inoculation in taken
d. Size or volume of inoculums
e. Environmental factors like temperature.
f. Number of viable ( living) organisms in the inoculum.
When conditions are suitable, division begins and after an acceleration in
rate of growth, the cells enter logarithmic (log) or Exponential phase

8. Logarithmic (Exponential) phase
• During the exponential (log) phase, microorganisms are growing and
dividing at the maximal rate possible given their genetic potential, the
nature of the medium, and the environmental conditions.
• Their rate of growth is constant during the exponential phase; that is, they
are completing the cell cycle and doubling in number at regular intervals .
• In logarithmic phase the bacterial cell start dividing and their number
increase by geometric progression with time - one cell splits to make two,
each of these cells divide to form 4 cells and so on.
• Thus the population doubles in number during a specific length of time
called the generation (doubling) time .

9. Logarithmic (Exponential) phase
• This phase is named as log phase because the logarithm of the
bacterial biomass increases linearly with time.
• This growth phase is also called as the exponential growth
phase because the number of cells is increasing as an
exponential function of time.
• Exponential (logarithmic) growth is balanced growth. That is, all
cellular constituents are manufactured at constant rates relative
to each other.
• The population is most uniform in terms of chemical and
physiological properties during this phase;
• Therefore exponential phase cultures are usually used in
biochemical and physiological studies.

10. Logarithmic (Exponential) phase
• If a bacterial culture in the log phase is inoculated into an identical fresh
medium , the lag phase is usually bypassed and exponential growth
continues. This happens because bacteria are already actively carrying out
the metabolism necessary for continued growth.
• However, if the chemical composition of the new medium differs
significantly from that of the original growth medium, the bacteria go
through a lag phase, wherein they synthesize the enzymes required for
growth in the new medium and then enter the logarithmic growth phase.
• If nutrient levels or other environmental conditions change, unbalanced
growth results.
• During unbalanced growth, the rates of synthesis of cell components vary
relative to one another until a new balanced state is reached. Once the
cells are able to grow again, balanced growth is resumed and the culture
enters the exponential phase.

11. Logarithmic (Exponential) phase
• In log phase , during this periods.
• a. Bacteria have high rate of
metabolism
• b. Bacteria are more sensitive to
antibiotics
• c. Rate of penetration of the medium
depends on the concentration of
material in the media.

12. Logarithmic (Exponential) phase
• During this phase generation (doubling)
time is constant for given bacterial
species grown under the same of
condition but varies among species.
• Doubling time does not change until
nutrients become depleted or toxic
metabolic products begin to
accumulate.
• However , when conditions for growth
become less favorable, Generation time
increases and eventually growth stops.
Log
Growth

13. Logarithmic (Exponential) phase
• For example : the G.T. of Pseudomonas may be as brief as 14 minutes.
The G.T. of M. tuberculosis as long 24 hours. This can be illustrated
with a simple example.
• Suppose that a culture tube is inoculated with one cell that divides
every 20 minutes .
• The population will be 2 cells after 20 minutes, 4 cells after 40
minutes, and so forth.
• Because the population is doubling every generation, the increase in
population is always 2n where n is the number of generations.
• The resulting population increase is exponential—that is, logarithmic.
• These observations can be expressed as equations for the generation
time.

15. Logarithmic (Exponential) phase
• The mean generation time (g) can be determined
directly from a semi logarithmic plot of the
growth data and the growth rate constant
calculated from the g value. The generation time
also may be calculated directly from the previous
equations.
• For example, suppose that a bacterial population
increases from 103 cells to 109 cells in 10 hours.
• Generation times vary markedly with the species
of microorganism and environmental conditions.
They range from less than 10 minutes (0.17
hours) to several days .
• Generation times in nature are usually much
longer than in culture.

16. Bacterial Doubling Time
• Escherichia coli 20 minutes
• Mycobacterium tuberculosis 18 hours
• Mycobacterium leprae 14 days

17. Logarithmic or Exponential Growth
The population of bacterial cells divide at a constant rate so that the
total number of cells doubles with each division

18. Bacteria
Undergo
Exponential
Growth

19. Stationary phase
• In a closed system such as a batch culture, population growth
eventually ceases and the growth curve becomes horizontal.
• This stationary phase usually is attained by bacteria at a population
level of around 109 cells per ml.
• In stationary phase after some time
1. Growth rate becomes stationary and there is a balance between cell
growth and division and cell death.
2. Rate of multiplication and death becomes almost equal
It may be due to
• a. Depletion of essential nutrients
• b. Accumulation of toxic products, inhibitory products of microbial
metabolism and sporulation may occur .

20. Stationary phase
• Generally , stationary phase cells are more resistant to
adverse physical conditions such as change in pH, radiation
or increased heat than exponential phase cells.
• There are a number of genes that are not expressed during
exponential growth but they get expressed in
microorganisms as they enter the stationary phase.
• Included among these are the genes for secondary
metabolites (e.g. antibiotics ) and genes for survival (sur) ,
which are indispensible to the survival of an organism
entering the stationary phase.

21. Decline or death phase
• Eventually the number of viable bacterial cells begins to decline due
to death of cells , signalling the onset of the death phase.
• However , the rate of the death phase need not be equal to the rate
of growth during the exponential phase.
• During this phase, bacterial cells may persist for some time as they
may tolerate the ever increasing accumulation of toxic wastes .
• In this period, dead cells will lyse from naturally occurring autolytic
enzymes and release cellular proteins , which may serve as nutrients
for the remaining viable cells.
• The factors responsible for are:
a. Nutritional exhaustion
b. Toxic accumulation
c. Autolysis enzymes.

22. Stages of Bacterial growth