factorsaffectingbacterialgrowth-

1. Factors affecting the growth of
bacteria

2. What is bacterial growth?
• Growth of bacteria is defined as an increase in the number of bacteria in a
population rather than in the size of individual cells.
• The growth of a bacterial population occurs in a geometric or exponential
manner: with each division cycle (generation), one cell gives rise to 2 cells,
then 4 cells, then 8 cells, then 16, then 32, and so forth.
• Thus, bacterial growth is proliferation of bacterium into two daughter
cells, in a process called binary fission.

3. Factors affecting bacterial growth
• Growth of bacteria is affected by many factors
such as nutrition concentration and other
environmental factors.
• Some of the important factors affecting bacterial
growth are:
• Nutrition concentration
• Temperature
• Gaseous concentration
• pH
• Ions and salt concentration
• Available water

4. 1. Nutrient concentration
• If culture media is rich in growth promoting substance,
growth of bacteria occurs faster. Decrease in nutrient
concentration decreases the growth rate.
• Different bacteria have different nutritional requirement.
• The relationship between substrate concentration
(nutrition) and growth rate is shown in figure.
Figure: Nutrient vs Growth rate
With increase in concentration
nutrition, growth rate of bacteria
increases up to certain level and
then growth rate remains
constant irrespective of nutrition
addition.

5. • micronutrient: an element or nutrient required in small
quantities.
• macronutrients: any element or nutrient required in large
amounts.
• The common nutrients which are found to be required in all
living things include carbon, nitrogen, sulfur, phosphorus,
potassium, magnesium, calcium, oxygen, iron and
additional trace elements.
• Both and macro- and micro-nutrients are critical in proper
organismal growth as they play important roles in cellular
and metabolic processes.
1. Nutrient concentration

6. 2. Temperature
• Temperature affects the growth of bacteria by various ways.
• The lowest temperature that allows the growth is called minimum
temperature and the highest temperature that allows growth is
called maximum temperature.
• There is no growth below minimum and above maximum
temperature.
• Below minimum temperature cell membrane solidifies and
become stiff to transport nutrients in to the cell, hence no growth
occurs.
• Above maximum temperature, cellular proteins and enzymes
denatures, so the bacterial growth ceases.
• The relationship between temperature and growth rate is shown
in figure below.
When temperature is
increases continuously from
its minimum, growth rate of
bacteria increases because
the rate of metabolic reaction
increases with increase in
temperature.
At certain temperature the
growth rate become
maximum, this temperature
is known as optimal
temperature.
On further increasing the
temperature above optimal,
growth rate decreases
abruptly and completely
ceases with reaching
maximum temperature.

7. • Increase in temperature will
increase enzyme activity. But if
temperatures get too high,
enzyme activity will diminish
and the protein (the enzyme)
will denature.
• Lowering temperature will
decrease enzyme activity.
• At freezing temperatures
enzyme activity can stop.
• Repeated cycles of freezing and
thawing can denature proteins.
In addition, freezing causes
water to expand and also forms
ice crystals, hence cells begin
to rupture.
Organisms can be classified according
to their optimum growth
temperature.:
PSYCHROPHILES grow best between
0oC and 20oC,
MESOPHILES grow best between 20oC
and 45oC and
THERMOPHILES grow best at
temperatures above 45oC.
2. Temperature

8. 3. pH
• pH affects the ionic properties of bacterial cell so it affects
the growth of bacteria.
• Most of the bacteria grow at neutral pH (6.5-7.5). However
there are certain bacteria that grow best at acidic or basic
pH.
• Relationship between pH and bacterial growth is given in
figure below.
Figure: pH vs Growth rate
Neutrophiles- neutral pH values (6.5 - 7.0),
eg- Escherichia coli, staphylococci,
and Salmonella spp.
Acidophilus- grow optimally at pH less than
5.55 Eg. Sulfolobus spp.
Alkaliphiles- grow best at pH between 8.0
and 10.5. Vibrio cholerae,

9. 4. Ions and salt
• All bacteria requires metal ions such as K+, Ca
++, Mg++, Fe++, Zn++, Cu++,Mn++ etc to
synthesize enzymes and proteins.
• Most bacteria do not require NaCl in media
however they can tolerate very low
concentration of salt.
• There is some halophilic bacteria such
as Archeobacteria that require high
concentration of salt in media.

10. 5. Gaseous requirement
• Oxygen and carbon-dioxide are important gases
that affects the growth of bacteria.
• Oxygen is required for aerobic respiration and
obligate aerobic bacteria must require O2 for
growth. Eg. Mycobacterium, Bacillus
• For anaerobes - Oxygen is harmful or sometime
lethal. However facultative anaerobes can
tolerate low concentration of O2.
• Carbon-dioxide is needed for capnophilic
bacteria. Such as Campylobacter, Helicobacter
pylori

11. 5. Gaseous requirement

12. • OBLIGATE AEROBES rely on aerobic respiration for ATP and they therefore
use oxygen as the terminal electron acceptor in the electron transport
chain. Eg- Pseudomonas
• MICROAEROPHILES require O2 for growth but they are damaged by
normal atmospheric levels of oxygen and they don't have efficient ways to
neutralize the toxic forms of oxygen such as superoxide (O2-) and hydrogen
peroxide (H2O2). Eg- Streptococci
• OBLIGATE ANAEROBES will die in the presence of oxygen because they
lack enzymes like superoxide dismutase and catalase. Superoxide
dismutase catalyzes the following reaction:
• 2O - + 2H+ ----> H O + O and catalase catalyzes:
2 2 2 2
• 2H2O2 ---> 2H2O + O2
• Eg- Clostridium
• AEROTOLERANT organisms like Lactobacillus ferment and therefore do not
use oxygen, however they do tolerate it.
• FACULTATIVE ANAEROBES are capable of both fermentation and aerobic
respiration. Eg- Escherichia coli

13. 6. Available water
• Water is the most essential factor for bacterial
growth.
• Available water in the culture media
determines the rate of metabolic and
physiological activities of bacteria.
• Sugar, salts and other substances are dissolved
in water and are made available for bacteria.