Bacterial Growth, Reproduction and Physiology

Bacterial growth is an increase in the size and number of organisms. In the laboratory, bacterial growth can be seen in one of two main forms:

  1. Colony formation: These are the macroscopic products of 20-30 cell divisions of a single bacterium on solid media.
  2. Transformation of a clear liquid medium to a turbid suspension.
Bacterial Growth, Reproduction and Physiology

Bacterial Reproduction

Bacterial multiplication takes place by simple binary fission as follows:

  1. The cell grows in size, usually elongates.
  2. The bacterial chromosome acts as a template for the replication of another copy.
  3. Each copy becomes attached to a mesosome on the cytoplasmic membrane.
  4. The protoplasm becomes divided into two equal parts by the growth of a transverse septum from the cytoplasmic membrane and cell wall. In some species, this septum splits the parent cell completely into two separate daughter cells. In others, the cell walls of the daughter cells remain continuous for some time after division giving the characteristic arrangement, e.g. pairs, clusters or chains.

Generation Time (Doubling Time): 
It is the time between two successive divisions. It may be as short as 13 min. in Vibrio cholerae and may reach 24 hours in Mycobacterium tuberculosis.

Growth Requirements

In order to grow and divide, bacteria need the following growth requirements:

1. Nutrients: 

According to the means by which a particular organism obtains energy and raw material to sustain its growth, bacteria are classified into:

a. Autotrophs
They can utilize simple inorganic materials, e.g. CO₂ as a source of carbon and ammonium salts as a source of nitrogen. They can synthesize complex organic substances from the simple inorganic materials. 

The energy required for their metabolism is predominantly derived from light or simple chemical reactions. Autotrophs are of no or little medical importance.

b- Heterotrophs
These bacteria, on the other hand, require organic sources for carbon, as they can not synthesize complex organic substances from simple inorganic sources. 

Most bacteria of medical importance are heterotrophic.

2. Oxygen (O2): 

According to O2 requirements, bacteria are classified into:

  • Strict or obligate aerobes require oxygen for growth, e.g. Pseudomonas aeruginosa.
  • Strict or obligate anaerobes require complete absence of oxygen, e.g. Bacteroides fragilis.
  • Facultative anaerobes generally grow better in presence of oxygen but still are able to grow in its absence, e.g. staphylococci, E. coli,...etc.
  • Micro-aerophilic organisms require reduced oxygen level, e.g. Campylobacter and Helicobacter.
  • Aerotolerant anaerobes have an anaerobic pattern of metabolism but can tolerate the presence of oxygen because they possess superoxide dismutase e.g. Clostridium perfringens.

Respiration and energy production: 

The cellular respiration is another name of glucose catabolism. When it takes place in presence of oxygen, it is called aerobic cellular respiration. When it takes place in absence of oxygen, it is called anaerobic cellular respiration.

Aerobic cellular respiration: 

The glucose catabolism under aerobic conditions results in the production of energy in the form of 38 ATP molecules. 

The final electron acceptor is molecular O2. During this type of respiration, superoxide (O₂") and hydrogen peroxide (H2O2) are formed. 

These molecules are highly toxic. To cope with this, aerobic organisms have developed two enzymes, superoxide dismutase and catalase, which detoxify these molecules.

Anaerobic cellular respiration:

It occurs in the absence of oxygen.

The final electron acceptor is an inorganic molecule such as nitrate (NO3), sulfate (SO42), or CO2.
The net yield of ATP molecules is less than it is with aerobic cellular respiration because nitrate, sulfate, and CO2 are not as good at accepting electrons as oxygen.

Compared to aerobes, obligate anaerobes lack superoxide dismutase and catalase and so they can not grow in presence of O2.


It is an anaerobic process, because it takes place in the absence of oxygen.

It is used by facultative anaerobes when they exist in an environment that does not contain a suitable inorganic final electron acceptor (NO3, SO42 or CO2).

This is the least efficient method of generating energy.

3. Carbon dioxide (CO2):

The minute amount of CO2 present in air is sufficient for most bacteria.
However, certain species require higher concentrations (5-10%) of CO₂ for growth (capnophilic) e.g. Neisseria spp. and Brucella abortus.
However, bacteria that do not grow in the presence of CO2 are called microaerophiles.

4. Temperature:

Mesophiles are organisms able to grow within a temperature range of 20-40°C.

Pathogens which replicate on or in human body are able to grow within this range, with an optimum temperature of 37°C which is the normal body temperature.

  • Psychrophiles (cold-loving): are capable of growth at refrigeration temperature (0- 8°C), e.g. Flavobacterium spp.
  • Thermophiles (heat-loving): grow best at high temperature (>60°C), e.g. Bacillus stearothermophilus.

Hydrogen ion concentration (pH):

Most microorganisms of clinical significance grow best in media whose pH is close to that of human body (pH 7.2). However, some microorganisms grow better at an alkaline pH (8-9), such as V. cholerae. Others, such as lactobacilli, prefer media of acidic pH (4 or less).

Growth phases (bacterial growth curve): 

If a small number of an organism is placed in a stable fluid nutrient medium under appropriate physical and chemical conditions, then the number of viable cells per milliliter is determined periodically and plotted a characteristic growth curve with four phases.
bacterial growth curve

  1. Lag phase: The initial number of bacterial cells remains constant. During this period, the cells adapt to their new environment. Enzymes and intermediates are formed to permit growth.
  2. Exponential (logarithmic) phase: There is marked increase in cell number and its rate is accelerated exponentially with time giving a characteristic linear plot on a logarithmic scale. In this phase, the organism shows typical morphology.
  3. Stationary phase: Exhaustion of nutrients and accumulation of toxic products cause growth to decrease. There is slow loss of cells through death which is just balanced by formation of new cells through growth and division. The number of viable bacteria remains constant.
  4. Decline phase: At the end of the stationary phase, the death rate increases and exceeds the multiplication rate due to nutrient exhaustion and accumulation of toxic metabolic end products. So, the number of viable bacteria decreases.
Bacterial Growth, Reproduction and Physiology
Dr.Tamer Mobarak


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