Factors Affecting Bacteria Growth

Bacteria constitute the largest form of life on earth. They are found everywhere; in bodies of living or dead animals and plants, all forms of organic matter, in water, dust, soil, and air. While some bacteria are useful to man, many are extremely harmful. Those that are not usually harmful to man at the sites they inhibit are called Commensals, while harmful ones are called Pathogens. Knowledge of conditions that affect bacteria growth is important in the maintenance of food hygiene.

Air and darkness

Bacteria growth is affected by darkness or reduced light. Bright light slows down the living or metabolic activities of bacteria. Also, most bacteria need air to survive while some can metabolize in the absence of oxygen. Those that require oxygen to live actively are called Aerobic bacteria (aerobes) e.g. Staphylococci and Salmonella, while those that survive without oxygen are known as anaerobes. E.g. Clostridium Botulinum and Clostridium Perfringens.

Food

Living organisms require food substances to metabolize and give energy. Bacteria as living organisms are by no means different. Incidentally, food spoilage or food poisoning organisms thrive best on foods that are most often preferred by human beings such as meat, eggs, milk, and their products. Therefor protein-based foods are the most favorable media for bacteria metabolism provided that environmental conditions are conducive.

PH (Acidity or Alkalinity)

Bacteria thrive actively at neutral PH of 7.0. Which is exactly the PH of the human body. The implication is that the human body is a conducive environment for bacterial growth.

Temperature

Each kind of bacterium has a peculiar temperature at which its activity is greatest (known as its optimum temperature). Bacteria can be grouped into three based on three different temperature ranges in which they can survive or metabolize. They are
Mesophylls- they thrive at an optimum temperature of 37^oC   or 96.8^oF (body temperature). They cause food poisoning and are responsible for food spoilage. 
Psychrophilic bacteria grow below 20^oC. Some cause food spoilage at temperature of the domestic refrigerator.
Thermophiles bacteria grow best at quite hot temperatures of up to 60^oC (140^oF). Thermophiles are prolific in food canning.

Time

Bacteria multiply rapidly but still, they need certain time to grow sufficiently to cause illness in man. Under ideal conditions one bacterium divides into two every 20 to 30 minutes.

Moisture or water

Bacteria like all living organisms need water for metabolism. Most cooked foods contain enough moisture to stimulate bacteria growth. However, bacteria cannot multiply in dried or dehydrated foods, rather they remain dormant till they gain moisture or that water is added and they revive again. Similarly water has a higher concentration of dissolved substances than the bacteria cytoplasm. Hence it causes water to flow out of the bacteria by the principle of osmosis, as a result they get dehydrated. 
Based on the above principle, some foods are dried while salt and sugar are added as a food preservation method. However certain bacteria spores can remain dormant in dehydrated conditions but may revive to vegetative form when favorable conditions return. Foods that have high water content such as creams, sauces, jellies and custards are ideal for the growth of bacteria.