The Most Important External Factors that Influence Bacterial Growth are listed below:
Bacterial physiology and biochemistry are studied by observing cultures grown in the laboratory on artificially prepared nutrient media. Various external factors influencing bacterial growth are-food, moisture, hydrogen-ion concentration, oxygen, carbon-dioxide, temperature and light.
Bacterial growth is to large extent dependent on an adequate supply of suitable food material, the specific nutrient requirements vary from species to species. The important nutrient requirements are carbon, nitrogen, inorganic salts and for certain species, accessory growth factors of bacterial vitamins.
For bacterial growth moisture is essential. Drying in the air damages bacteria.
3. Hydrogen-ion-concentration of pH:
Most of the microbes grow better at a slightly alkaline pH (pH 7.2-7.6). Some acidophilic bacteria flourish in acidic pH. Those needing strong alkaline medium are termed basophilic.
4. Oxygen needs:
Most bacteria can grow in the presence of oxygen and air and also in its absence. Those which grow in the presence of oxygen are called aerobes, while those which grow in its absence are termed anaerobes.
Those which can grow under both the conditions are called facultative anaerobes, hereas bacteria that can grow in complete absence of oxygen are named obligatory anaerobes.
All bacteria need the presence of small amounts of CO2 for growth, an amount provided by atmosphere or by the metabolic reactions occurring in the bacteria itself. However, some bacteria need a higher concentration of CO2 (5-10%).
For bacteria there is a range of temperature at which growth can occur. So there is a maximum, a minimum and the intermediate optimum temperature (at which the growth is most rapid). In the laboratory this optimum temperature is maintained in an incubator thermostatically controlled.
Majority of bacteria grow between 24°C and 40°C and are termed mesophilic. 30°C is optimal for free living and 37 °C is optimal for parasites in man or animals. Bacteria that grow best between 60-70°C are called thermophilic, while those growing best between 15-20°C are labelled as psychromphilic.
Darkness is favourable condition for growth and viability of bacteria. Direct sunlight is injurious to bacterial growth. Some bacteria can produce pigmentation on exposure to light and are called as photochromofrens.
8. Symbiosis or mutual beneficial coexistence:
A living organism multiplying in a human body is called as a parasite and the person harboring is the host.
When both the parasite and the host derive benefit from each other—it is termed symbiosis. Certain intestinal bacteria provide vitamins to their host without causing any pathogenic effect-a symbiotic relationship.