Bacterial Growth
and Physiology
• Growth: increase in size of organisms and
increase in their number, the net effect is
increase in the total mass of the culture.
• In the laboratory: growth is need as technique
for detection and identification and for
assessment of antibiotic effects.

Types of growth:
1- development of colonies, the
macroscopic product of 20-30 cell
divisions of a single cell.
2- by transformation of clear broth medium
to a turbid suspension
Media for bacterial growth:
→ could be solid (agar) or liquid (broth)
→the aim is achievement of pure culture;
only one type of growing cell and single
colonies of this type.
Media could be :
1. Nutrient
2. Supportive
3. Selective or enrichment
4. Examples on media :
Blood agar, MacConkey agar and Chocolate
agar
Bacterial Nutrition
Bacteria need nutrients for
1- Maintenance of their metabolism
2- Cell division.
Sugars, proteins and lipid macromolecules are
broken down outside the cell into smaller
molecules that are transported across the
membrane,
in the cytoplasm the metabolite are converted into
pyruvic acid which is either used for energy or to
synthesis of new carbohydrate, protien and lipid ,
and nucelic acid
Elements needed:
• 1. Hydrogen and Oxygen------obtained from Water
• 2. Carbon Source: external source must be found e.g.
carbohydrate (sugars) degraded by either oxidation
or fermentation and this provide ATP
• 3. Nitrogen source: main source is ammonia or
produced by the bacterium by deamination of amino
acids released from proteins.
• 4. Minerals
• 5. Growth factors: that a cell must have for
growth but cannot synthesize itself. Such as
amino acids, purines, pyrimidines, and
vitamins
• Nutrient uptake by diffusion and active
transport.
 In this lecture, we will talk about:
 Growth requirement of the bacteria:
o Nutrients
o Growth factors
o Oxygen
o Carbon dioxide
o Temperature
o Hydrogen ion concentration
 Bacterial growth curve
Nutrients
Autotrophic bacteria
Heterotrophic bacteria
o Use simple inorganic materials
like CO2 as a source of carbon and
ammonium as a source of nitrogen
o Require complex organic
materials
o They form complex organic
compounds from the simple
inorganic materials
o Can not form organic compounds
from the simple inorganic materials
o Derive their energy:
From light (photosynthetic
bacteria)
Oxidation of inorganic materials
(chemolithotrophic bacteria)
o Derive their energy:
Oxidation or fermentation of
organic compounds such as glucose
Most bacteria of medical importance
are heterotrophic bacteria
Growth factors
 These are organic compounds which bacteria must contain
to grow.
 But, they are unable to synthesize them
 So, they must be added ready formed to the culture
medium
 Examples: amino acids, vitamins, purines and pyrimidines
Gas Requirements
Bacteria are classified according
to O2 requirement into
1- aerobic
2- obligate anaerobic
3- facultative anaerobic
4- microaerobic
Aerobic and anaerobic bacteria can be identified by growing them in test
tubes of thioglycollate broth:
1- Obligate aerobes need oxygen because they cannot ferment or respire
anaerobically. They gather at the top of the tube where the oxygen
concentration is highest.
2- Obligate anaerobes are poisoned by oxygen, so they gather at the bottom
of the tube where the oxygen concentration is lowest.
3- Facultative anaerobes can grow with or without oxygen because they can
metabolise energy aerobically or anaerobically. They gather mostly at the top
because aerobic respiration generates more ATP than either fermentation or
anaerobic respiration.
:4- Microaerophiles need oxygen because they cannot ferment or respire
anaerobically. However, they are poisoned by high concentrations of oxygen.
They gather in the upper part of the test tube but not the very top.
:5- Aerotolerant organisms do not require oxygen as they metabolise energy
anaerobically. Unlike obligate anaerobes however, they are not poisoned by
oxygen. They can be found evenly spread throughout the test tube
Aerobic bacteria produce three enzymes in order to
protect themselves from oxygen-free radicals which are
highly toxic. These enzymes: superoxide dismutase ,
catalase peroxidase
2O2- + 2H+
2H2O2
H2O2+AH2
SOD
Catalase
H2O2+O2
2H2O +O2
peroxidase
2H2O+A
Oxygen
Class
Definition
Examples
Obligate aerobe
Grow only in the
presence of O2
Obligate anaerobe
Can not grow in
the presence of O2
Facultative anaerobe
Can grow in the
Most bacteria of
presence or absence medical importance
of O2
Require low O2
Campylobacter
tension
Microaerophilic
bacteria
Mycobacterium
tuberculosis
Clostridia
2H 2O  2O2  2H 2O2  O2
In the presence of oxygen, two toxic substances to the bacteria
are produced which are hydrogen peroxide and superoxide anion.
In obligate aerobes and facultative anaerobes:
Catalase and peroxidase enzymes degrade hydrogen peroxide.
Superoxide dismutase enzyme degrades superoxide anion.
BUT
In obligate anaerobes:
These enzymes are not present.
So, the presence of oxygen is toxic to them.
Carbon dioxide
 Most bacteria require CO2 in small concentration as that
in the air.
 However, some bacteria require higher concentrations
of CO2.
 These bacteria are called capnophilic bacteria.
 Example:
 Neisseria requires 5-10% CO2
CO2
• N. meningitidis
• N. gonorrhoeae
Temperature
Range of
temperature
Optimum
temperature
Mesophilic
18 - 42
37
Psychrophilic
5 - 30
15 - 20
Thermophilic
25 - 80
50 - 60
Bacteria
Most bacteria of medical importance
are mesophilic bacteria
Temperature
• Psychrophilic forms (15-20°C)
• Mesophilic forms (30-37 °C) include all
human pathogens and opportunists.
• Thermophilic forms (50-60 °C)
Hydrogen ion concentration
 Hydrogen ion concentration is called pH.
 Most bacteria grow within pH range 7.2-7.6
 However, some bacteria require different pH.
 Examples:
 Vibrio cholera needs alkaline pH
 Lactobacilli need acidic pH
pH
• Neutrophiles (6.5-7.5) almost all pathogen
• Acidophiles (3-5) ex. Lactobacillus
• Alkaliphiles (pH > 8) ex. Vibrio cholera
Bacterial Growth
• Bacteria multiply by binary fission
Bacteria growth (reproduction)
 Binary fission:
 The bacterial cell divides into two equal daughter
cells similar in genetic characters to the mother cell.
1)
2)
3)
4)
Elongation of the bacterial cell.
Duplication of the chromosome.
The two sister chromosomes attach to the mesosome.
Cell membrane and cell wall will form a transverse septum
that divides the mother cell into 2 equal daughter cells.
5) The daughter cells may remain attached to each other.
 Doubling time: the time required by the
bacterial cell to double its number.
BACTERIAL REPRODUCTION
Bacterial Growth
• Bacteria undergo exponential growth
(Logarithmic growth):
Number of cells
1 2 4 8
16
• Generation : the time it takes for a population
of bacteria to double in number
• many common bacteria ：20~60 min
• most common pathogens in the body： 5-10 hours
Bacterial growth curve
 A small number of bacteria is inoculated in a liquid
medium and the number of living bacteria is counted
at intervals.
 Growth curve with 4 phases is obtained:
1) Lag phase:
 No cell division.
 The bacteria form the enzymes and molecules
needed for replication.
 Clinical significance: this phase = incubation
period of a disease.
2) Logarithmic phase:
 Rapid cell division occurs.
 The number of living bacteria increases by time.
 Clinical significance: this phase = symptoms and
signs of the disease.
3) Stationary phase:




Nutrients are exhausted.
Waste products are accumulated.
The number of dying cells = number of new cells.
The number of living bacteria remains constant.
4) Decline phase:





Nutrients are more exhausted.
Waste products are more accumulated.
The number of dying cells > number of new cells.
The number of living bacteria decreases by time.
Clinical significance: this phase = recovery and
convalescence.
the growth curve:
• a. The Lag Phase: The lag phase represents a period
during which the cells, adapt to their new environment.
High metabolic activity occurs but cell do not dived.
Enzymes and intermediates are formed and accumulate
until they are present in concentrations that permit
growth to resume.
• b. The Log or Exponential Phase:rapid division and the
mass increases in an exponential manner. This continues
until one of two things happens: either one or more
nutrients in the medium become exhausted, or toxic
metabolic products accumulate and inhibit growth.
the growth curve
• c. The Maximum Stationary Phase: nutrient
depletion or toxic product cause growth to slow
until number of new cells produced balances the
number of cells that die, resulting in a steady
state ( equilibrium between cell division &
death)
• d. The Death or decline Phase: decline in the
number of viable bacteria. Number of death
exceeds the number of new cells formed
Table 3. Phases of the Microbial Growth Curve
Phase
Lag
Growth Rate
Zero
Exponential
Constant
Maximum stationary
Zero
Decline
Negative (death)
GOOD LUCK