Factors Affecting Microbial Growth in Food:

(a). Intrinsic Factor

These are factors that exist as part of the food product itself. For example, fish have certain characteristics that may promote the growth of microorganism. The common intrinsic factors that affect the growth and multiplication of microorganisms in foods are: pH, water activity, oxidation reduction potential, nutrient content, antimicrobial contents, biological structure.

(b). Extrinsic Factor

This are factors in the environment external to the food, which affect both the microorganisms and the food itself during processing and storage. Extrinsic factors include temperature, humidity and gases.

Extrinsic Factor


• The growth of microorganisms is affected by the environmental temperatures.

• Various microorganisms are able to grow at certain temperatures and not others.

• Bacteria can therefore be divided into the following groups depending upon their optimum temperature of growth includes: Psychrophilic, Mesophilic, Thermophilic bacteria.

1. Psychrophilic Bacteria

These are microorganisms that grow at low temperatures (0-20°C), 15°C optimum. Example; Bacillus Pschrophilus etc. psychrophiles are the major course of refrigerated food spoilage.

2. Mesophilic Bacteria

They grow best at room temperature. They have optima around 20- 45°C and often have a temperature minimum of 15 to 20°C and a maximum of about 45°C. Most human pathogens fall under this group because of the normal 37°C body temperature.

3. Thermophilic Bacteria

They grow at high temperatures (Between 55°C and 85°C), optima between 55°C and 66°C. Hyperthermophiles are those organisms that usually have optima between 85°C and about 113°C. Temperature is of paramount importance in food safety because if the growth temperature ranges for dangerous microorganisms are known, it helps in employing appropriate production temperature and time for foods that require heating. It also aids in selecting the proper temperature for food storage to make them less able to grow and reproduce.


Some microorganisms require oxygen in order to grow and multiply. Such organisms are called aerobic microorganisms. An example is Escherichia coli; On the other hand, there are some microorganisms that grow without oxygen, called anaerobic microorganisms. For example Clostridium botulinum, this bacterium causes botulism in very low oxygen environments as is in canned foods. Obligate aerobe: are those that completely depends on atmospheric oxygen for growth e.g. protists and fungi.

Organisms can be classified based on oxygen requirements as follows;

Facultative anaerobe: are those that do not require oxygen for growth but grows better in its presence e.g. Escherichia, Enterococcus.

Aerotolerant anaerobe: grow equally well in the presence of oxygen e.g. Streptococcus pyogenes.

Obligate anaerobe: does not tolerate oxygen and dies in its presence e.g. Clostridium, Bacteroides

Obligate aerobe: grow only in the presence of oxygen.

Microaerophile: requires oxygen level between 2 10% for growth and is damage by atmospheric oxygen levels (20%) e.g. Campylobacter, Spirillum volutans.


An important factor for the growth of microorganisms at the food surfaces is the humidity of the storage environment. Dry conditions are devoid of water for microbial activities referred to as water activity and thus better for food storage than moist conditions. Foods stored in a dry atmosphere, therefore, have a longer shelf life than foods stored in a humid environment. For example, dry grains stored in an environment with high humidity will take up water and undergo mould spoilage.