Microbial growth is characterized by an increase in cell number or size, typically studied in phases: lag, log, stationary, and death. During the log phase, cells multiply exponentially, while environmental factors lead to stationary and death phases through nutrient limitation and toxic accumulation. The generation time and mean growth rate constant can be calculated to understand population dynamics in microbiology.

1. Kalavati Prajapati

2. Increase in cellular constituents that may
result in:
– increase in cell number
• e.g., when microorganisms reproduce by budding
or binary fission
– increase in cell size
• e.g., coenocytic microorganisms have nuclear divisions that are
not accompanied by cell n divisions
• microbiologists usually study population growth rather than
growth of individual cells

3. Definition: Growth may be defined as an increase in
cellular constituents. It leads to a rise in cell number
when microorganisms reproduce by processes like
budding or binary fission.

4.  The growth of microorganisms reproducing by
binary fission can be plotted as the logarithm
of the number of viable cells versus the
incubation time
 The resulting curve has four distinct phases
1. Lag Phase (Adaptation Phase)
2. Log Phase (Exponential Phase)
3. Stationary Phase (idio phase)
4. Death Phase (Decline phase)

7.  Cell division does not take place
 Cell synthesizing new components
 – e.g., to replenish spent materials
 – e.g., to adapt to new medium or other
conditions
 Varies in length
 In some cases can be very short or even
absent

9.  Microorganisms are growing and dividing
at the maximal rate
 The population is most uniform in terms of
chemical and physiological properties
 Exponential growth is balanced growth.

12.  Usually is attained by bacteria at a population
level of around 109
cells per ml.
 Microbial populations enter the stationary
phase for several reasons.
1.Nutrient limitation (Starvation):The more important
changes are in gene expression and physiology.
Starving bacteria frequently produce a variety of
starvation proteins, which make the cell much more
resistant to damage in a variety of ways.
2.O2 availability
3.Accumulation of toxic waste products
4.Critical population level is reached

13.  Morphological changes
– e.g., endospore formation,
decrease in size,
protoplast shrinkage,
nucleoid condensation
 Production of starvation proteins long-term
survival increased virulence

15.  Cells dying, usually at exponential rate
 e.g. environmental changes like nutrient
 deprivation and the buildup of toxic wastes lead
 Death
 – irreversible loss of ability to reproduce
 In some cases, death rate slows due to
accumulation of resistant cells

16.  Generation (doubling) time:
 time required for the population to double in size
 Mean growth rate constant:
 number of generations per unit time usually
expressed as generations per hour

17.  1--->2 --->4--->8--->16--->32--->64--->128
 2--->2¹--->2²--->2³-------------
 Assume 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 no of generation.
 Let N0 = the initial population number N
t = the population at time t
n = the number of generations in time t
Nt = N0 x 2n.
 Solving for n, the number of generations, where all logarithms are to the base 10,
log Nt= log N0+ n · log 2, and
n = log Nt - log N0 = log Nt - log N0
log 2 0.301
 The rate of growth during the exponential phase in a batch culture can be
expressed in terms of the mean growth rate constant (k).
This is the number of generations per unit time, often expressed as the generations
per hour.
k = n = log Nt - log N0
t 0.301 x t

18. During the
exponential phase
each microorganism is
dividing
at constant intervals.
Thus the population
will double in number
during a specific
length of time called
the generation time or
doubling time.

21.  The time it takes a population to double in size—that is, the
mean generation time or mean doubling time (g), can now be
calculated. If the population doubles (t g), then
Nt= 2 N0.
 Substitute 2N0 into the mean growth rate equation and solve for
R
R= log2N0- log N0 = log2 + log N0
0.301 g 0.301 g
k= 1/g
 The mean generation time is the reciprocal of the mean growth
rate constant. So
g=1/R
 The mean generation time (g) can be determined directly from a
semilogarithmic plot of the growth data.
 growth rate constant calculated from the g value.
 The generation time also may be calculated directly from the
previous equations.

22. 1. Lag-phase: a period of adaptation of inoculated cells to the new
environment; the number of live cells usually decreases,
2. Acceleration phase: cell begin to multiply at an increasing rate,
3. Exponential phase: the number of cells rises exponentially with
time,
4. Deceleration phase: multiplication rate decreases,
5. Stationary phase: multiplication rate is in equilibrium with
death rate,
6. Death phase: which may be subdivided into accelerated death
phase, exponential death phase and death declination phase.

23.  At home and dairy plants
 At microbiology Laboratory
 At hospital
 At fermentation industry

24. 1. The most common mode of cell division in bacteria is
a) Binary fission
b) Transverse binary fission
c) Longitudinal binary fission
d) None of these
2. Physiologically the cells are active and are synthesizing new protoplasm in
which stage of the growth in bacteria
a) Log phase
b) Lag phase
c) Stationary phase
d) None of these
Multiple Choice Questions

25. 3. The most active stage in the sigmoid curve of bacteria in which maximum
growth is attained
a) Lag phase
b) Stationary phase
c) Decline phase
d) Log phase
4. Log-phase is also known as
a) Death phase
b) Exponential phase
c) Lag-phase
d) None
5. The no. of generations per hour in a bacteria is
a) Growth rate
b) Generation time
c) Sigmoid curve
d) None of these

26. 6. A culture of bacteria produces 5 generations in 2 hours. What is the
generation time for this bacterium under those conditions.
a) 15 minutes
b) 24 minutes
c) 30 minutes
d) 75 hours
7. The no. of generations per hour in a bacteria is
a) Growth rate
b) Generation time
c) Sigmoid curve
d) None of these
8. The reproduction rate is equal to death rate in which stage
a) Decline phase
b) Stationary phase
c) Lag phase
d) Log phase

27. 9. If a single bacterium replicated every 30 minutes, how many bacteria would
be present in 2 hours?
a) 16
b) 32
c) 8
d) 4
10. During the lag phase:
a) cells are growing in number.
b) cells are engaged in intense enzymatic activity.
c) changes in pH occur.
d) nutrients are depleted.
e) cells are decreasing in number.

28.  Prescott, L.M., Harley, J.P., Klein, D.A. (2002). Microbiology. Fifth Edition. Wm. C.
Brown Pub. Dubuque, Iowa. pp. 112-125.
 Pelczar, M.J., Chan, E.C.S., Krieg, N.R.(1993) Introduction to Microbiology . Fifth
Edition. Tata Mc-Graw – Hill Edition.pp-119-132
 Ingraham, J.L. and Ingraham C. A(2008) Introduction to Microbiology. Third Edition
 Modi H.A. (2014) A handbook of Elementary Microbiology .Shanti Prakashan. pp-203-
216
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