Physical Parameters For Bacterial Growth: Apart from the type of media for bacterial growth many physical conditions of the environment are very much essential for optimum growth. These physical conditions have to be studied for every type of microbe. Some of these conditions are temperature, pH, oxygen, light, hydrostatic pressure, etc.
Temperature is one of the most important physical factors that have a great influence on the growth of microorganisms. The temperature that allows for rapid growth during a short time is known as the optimum growth temperature. The highest temperature at which micro-organisms show growth is known as maximum growth temperature and the lowest temperature at which micro-organisms show growth is known as minimum growth temperature. Based on temperature tolerance and its influence on growth, bacteria may be classified into the following categories:
(i) Psychrophiles: Psychrophiles can grow at 0°C but have an optimum temperature of 15°C or lower (obligate psychrophiles). Psychrotrophic or facultative psychrophiles can also grow at 0°C, but grow best at 20 to 30°C (Table 2.4).
(ii) Mesophiles: Mesophiles are growing within the range of 20 to 40°C. All bacteria that are pathogenic for humans and warm-blooded animals are mesophiles.
(iii) Thermophiles: Thermophiles prefer high temperatures (45 to 70°C) for growth. The growth range of many thermophiles extends into the mesophilic region (37 to 55°C). These species are called facultative thermophiles. Some thermophiles that grow above 60°C, are called true or obligate, or steno thermophiles. Several mesophiles that can tolerate high temperatures (above 60°C) but do not grow at this temperature are called thermoduric.
Table.1: Temperature ranges for bacterial growth
The pH of the growth medium of bacteria has a profound effect on the multiplication of microorganisms. Each microbial species has a definite pH range for growth and multiplication. Depending on the optimum pH value of microorganisms, they can be classified as follows:
(i) Acidophiles: These micro-organisms have an optimum pH range between 1 to 6.5 e.g. Thiobacillus thiooxidans (optimum pH 2 to 3.5) Lactobacillus acidophilus (optimum pH 5.8 to 6.6).
(ii) Neutrophiles: Most bacteria grow best in a narrow pH range between 6.5 and 7.5, such bacteria are called neutrophiles e.g. Escherichia coli, Salmonella typhi, all pathogenic bacteria.
(iii) Alkalophiles: These micro-organisms have an optimum pH range between 7.5 to 14. e.g. Vibrio cholerae (optimum pH 9.0), and Agrobacterium species (optimum pH 12.0).
3. Gaseous requirements
The principal gases that affect bacterial growth are oxygen and carbon dioxide. The need for oxygen for a particular bacterium reflects its mechanism to meet the requirements of energy. Depending on oxygen requirement, bacteria can be classified as follows (Fig. 2.11).
(i) Aerobic bacteria: These bacteria require oxygen for growth and can grow when incubated in an air atmosphere (21% oxygen) e.g. Escherichia coli, Staphylococcus aureus.
(ii) Anaerobic bacteria: These bacteria do not use oxygen for energy and growth. Oxygen is toxic to such cells and they cannot grow when incubated in an air atmosphere. Some bacteria can tolerate low levels of oxygen and are called tolerant or non-stringent anaerobes. Some other bacteria that can not tolerate even low levels of oxygen and may die after brief exposure to air are called strict anaerobes. e.g. Clostridium species.
(iii) Facultatively anaerobic bacteria: These bacteria do not require oxygen for growth but if oxygen is available, are used for energy production. These bacteria grow in both aerobic and anaerobic conditions e.g. Pseudomonas species.
(iv) Microaerophilic bacteria: These bacteria require low levels of oxygen for growth but cannot tolerate the levels of oxygen present in an air atmosphere e.g. Campylobacter jejuni, Lactobacillus plantarum.
4. Osmotic pressure
Bacteria are more tolerant to osmotic variations because of the mechanical strength of the cell wall. They can grow in media with widely varying contents of salt, sugar, and other solutes. Sudden exposure of bacteria to solutions of high salt concentrations may cause plasmolysis. Hence, 0.5% NaCl is added to almost all culture media to make the environment isotonic.
Darkness is usually favorable for the growth and viability of all microorganisms. They are sensitive to ultraviolet radiation, direct light, and other radiations.
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