Microbial growth involves an increase in cell numbers through cell division and reproduction. There are four phases of microbial growth: lag phase as cells adapt to their environment; log or exponential phase of rapid cell division; stationary phase as resources are depleted and growth balances mortality; and death phase as cells die off. Microbial growth is influenced by environmental factors like pH, temperature, gas availability, and pressure.

1. Microbial
Growth
When microbes are provided with nutrients & required
environmental factors, they become metabolically active and
grow
1Lecturer:: Mohamed Ali. Bsc, M.Eng
College of Agriculture, University Of Hargeisa.

2. Chapter contents:
1- Bacterial Division/reproduction
2- Stages of Bacterial growth curve
3- Measurement of Microbial
growth
4- Environmental factors effect
microbial growth
2

3.  What is Microbial growth?
 involves an increase in the number of
cells rather than in the size of individual
cells.
 2 levels of growth:
1. A cell synthesizes new components,
increase its size
2. Increase number of cells in the
population
3

4.  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., some microorganisms have nuclear
divisions that are not accompanied by cell
divisions
 microbiologists usually study population
growth rather than growth of individual
cells
4

5. 1- The Bacterial division/reproduction
 Bacteria normally reproduce by a method
called binary fission
What is Binary Fission.?
Binary Fission is the process of bacteria
reproduction where one cell become two
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6. Stages of Binary Fission
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7. Generation Time (Doubling time)
 What is Generation
Time?
 The time required
for a cell to divide
(and its population
to double) is called
the generation time
 As you seen in the
picture, cell’s
division produces
two cells, two cells’
divisions produce
four cells, and so
on. 7

8. Bacterial Generation (Doubling) Time
 Examples’;
 Escherichia coli 20 minutes
 Mycobacterium tuberculosis 18 hours
 Mycobacterium leprae 14 days
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9. Bacteria Undergo Exponential Growth
9

10. cells are dividing and doubling in number at regular intervals
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11. 2-Stages in Bacterial Growth curve
 When a few bacteria are inoculated into a liquid growth
medium, it is possible to plot a bacterial growth curve
that shows the growth of cells over time (See picture).
There are four basic phases of growth: 1- the lag, 2- log
(Exponential) , 3- stationary, and 4-death phases.
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12. Stages of Bacterial growth curve
 Bacterial populations follow a sequential
series of growth phases: the lag, log,
stationary, and death phases.
 Knowledge of the bacterial growth curve is
critical to understanding population
dynamics and population control in the
course of infectious diseases, and in food
preservation .
12

13. no increase
maximal rate of division
and population growth
population growth ceases
decline in
population
size
4 Stages of Bacterial growth Curve
13

14. a) Lag Phase
 Bacteria are first introduced into an environment
or media culture.
 Bacteria are trying to adapt to nutrients
 Lag phase-preparing to grow in size and synthesize
enzymes etc.
 varies in length
 in some cases can be very short or even absent
Stages of Bacteria growth Curve
14

15. b) Log (Exponential) Phase
 also called log phase
 rate of growth is constant
 Population number of cells undergoing
binary fission doubles at a constant
interval called generation time
 Continue as long as cells have adequate
nutrients & good environment
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16. c) Stationary Phase
 If exponential growth continued
unchecked, startlingly large numbers of
cells could arise.
 In this stage microbial death is equal to microbial
growth e-g Death=growth.
16

17. Possible reasons for entry into
stationary phase
nutrient limitation
limited oxygen availability
toxic waste accumulation
critical population density reached
17

18. d) Death Phase
 Decline in the number of viable cells
 cells dying, usually at exponential rate
 Death; loss of ability to reproduce
 in some cases, death rate slows due to
accumulation of resistant cells
 Slower than log phase
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19. 3- Measurement of
Microbial Growth
 can measure changes in number of cells
in a population
 can measure changes in mass of
population
19

20. Measurement of Cell Numbers
 What are the lab equipment's used to
measure bacteria cell numbers?
 There are two types :-
 1- Direct cell counts:-
a) counting chambers
b) electronic counters
c) on membrane filters
 2- Viable cell counts
 plating methods (spread, pour plate)
 membrane filtration methods 20

21. a) Counting chambers
 easy, inexpensive, and quick
 useful for counting both eucaryotes and
procaryotes
 cannot distinguish living from dead cells
21

22. b) Electronic counters
 Such as Coulter counter
 microbial suspension forced
through small hole or orifice
 movement of microbe through
orifice impacts electric current
that flows through orifice
 cannot distinguish living from
dead cells
 quick and easy to use
 useful for large
microorganisms and blood
cells, but not prokaryotes
22

23. c) membrane filters
 cells filtered through special membrane that
provides dark background for observing cells
 cells are stained with fluorescent dyes
 useful for counting bacteria
 with certain dyes, can distinguish living from dead
cells
23

24. 4- Important environmental factors
affect microbial growth
1.pH
2.Temperature
3.Gas requirement
4.Pressure
5.And other environmental factors e-g
radiation, water activities etc.
24

25. 1. pH
 negative
logarithm of the
hydrogen ion
concentration
 Affect the activity
& integrity of
enzymes &
structural
components of a
cell
25

26. pH
 Optimum pH for most microbes ranges
approximately from 6 to 8
 acidophiles
growth optimum between pH 0 and pH 5.5
 neutrophiles
growth optimum between pH 5.5 and pH 7
 alkalophiles
growth optimum between pH8.5 and pH 11.5
26

27. 2. Temperature
 organisms
exhibit distinct
cardinal growth
temperatures
 minimum
 maximum
 optimum
27

28. 28

29.  Pychrophiles
 grow well at Oo
C
 optimum growth at 15o
C or lower
 (cold loving microbes )
 Cannot grow above 20o
C
 Psychrotroph
 can grow at 0-7o
C
 optimum between 20-30o
C,
 max around 35o
C
 Mesophiles
 growth optimum around 20-40o
C
 (moderate temp. loving microbes)
 Thermophiles
 Growth range is 45o
C and 80o
C
 optimum between 55-65o
C
 (heat loving microbes)
 Hyperthermophiles
 optimum growth between 80o
C and 100o
C
 May grow to temperatures of 120o
C
29

30. 30

31.  Depends the kinds of microorganism, some
microorganism need oxygen while others not
need oxygen to grow
3. Gas Requirements
31
Thioglycollate broth:
1. Aerobic (pseudomonas aeruginosa)
2. Facultative (Staphylococcus aureus)
3. Facultative (Escherichia coli)
4. Obligate Anaerobe (Clostridium
butyricum)

32. need
oxygen
prefer
oxygen
ignore
oxygen
oxygen is
toxic
Require
< 2 – 10%
oxygen
32

33. 4. Pressure
 Microbes obtain almost all their nutrients
in solution from surrounding water
 barophile organisms
Adapted to life under high pressure
require or grow more rapidly in the
presence of increased pressure
Example : bottom dwellers in the ocean
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34. Review Questions
1) What are the four stages of microbial growth? Explain
what happens each phase?
2) Name 3 direct methods use to measure microbial cell
numbers?
3) Name 2 methods used measure viable cell counts
4) What are the four environmental factors effect microbial
growth?
5) What is binary fission?
6) What is generation (doubling time)?
7) State 4 possible reasons why bacterial growth curve
entry into stationary phase?
8) How does pH effect microbial growth?
9) What is microbial growth? 34