Introduction:
Apart from nutritional requirements microbes need various physical parameters for their optimum growth.
Some important physical parameters needed for bacterial growth are discussed as follows,
Temperature:
Temperature has great influence on growth of microorganisms.
The temperature that allows maximum growth during a short period of time is known as the optimum growth temperature.
The highest temperature at which microbes show growth is known as maximum growth temperature.
The lowest temperature at which microbes show growth is known as minimum growth temperature.
Based on temperature tolerance bacteria are classified as follows.
Psychrophiles:
Grow at a temperature range of 0 ℃ to 20 ℃.
They are further classified as,
Obligate psychrophiles.
Facultative psychrophiles.
Mesophiles:
Grow at a temperature range of 20 ℃ to 40 ℃.
Most of the pathogenic microorganisms that belong to this class.
Some mesophiles can tolerate high degrees of temperature i.e. upto 60 ℃ but can not grow are called “Thermoduric”.
Thermophiles:
Grow at a temperature range of 45℃ to 70℃.
They are further classified as,
Obligate thermophiles.
Facultative thermophiles.
pH:
Most pathogenic bacteria grow between pH 7.2 and 7.6 they are called “Neutrophiles”.
Very few bacteria, such as lactobacilli, can grow at acidic pH below 4.0 they are called “Acidophiles”.
Many food items, such as pickles and cheese, are prevented from spoilage by acids produced during fermentation.
V. cholerae is an example of the bacteria that can grow at an alkaline (8.2–8.9) pH such bacteria are called “Basophils”.
Gaseous Requirements:
Oxygen Requirement:
On the basis of their oxygen requirements bacterias can be classified broadly into aerobic and anaerobic bacteria.
Aerobic bacteria:
They require oxygen for their growth. They may be;
Obligate aerobes:
They strictly require the presence of oxygen for their growth e.g. Pseudomonas aeruginosa.
Facultative aerobes:
They are ordinary aerobes but can also grow without oxygen (e.g., E. coli).
Most of the pathogenic bacteria are facultative aerobes.
Microaerophilic bacteria:
These bacteria can grow in the presence of low oxygen and in the presence of low (4%) concentration of carbon dioxide (e.g., Campylobacter jejuni).
Some fermentative organisms (e.g., Lactobacillus plantarum) are aerotolerant but do not contain the enzyme catalase or superoxide dismutase. Oxygen is not reduced, and therefore hydrogen peroxide (H2O2) and nascent oxygen (O2) are not produced.
Anaerobic bacteria:
Obligate anaerobes are the bacteria that can grow only in the absence of oxygen (e.g., Clostridium botulinum, Clostridium tetani, etc.).
These bacteria lack superoxide dismutase and catalase; hence oxygen is lethal to these organisms.
Facultative anaerobes can tolerate little amount of oxygen.
Carbon dioxide Requirement:
The organisms that require higher amounts of carbon dioxide (CO2) for their growth are called capnophilic bacteria.
They grow well in the presence of 5–10% CO2 and 15% O2.
In a candle jar, 3% CO2 can be achieved.
Examples of such bacteria include H. influenzae, Brucella abortus, etc.
Light:
Depending on the source of energy bacteria make use of, they may be classified as;
Phototrophs (bacteria deriving energy from sunlight)
Chemotrophs (bacteria deriving energy from chemical sources).
Osmotic pressure:
Microbes obtain almost all their nutrients in solution from surrounding water.
Hence factors such as osmotic pressure and salt concentration of the solution affect their growth.
Bacteria due to mechanical strength of their cell wall are able to withstand a wide range of external osmotic changes.
Organisms requiring high osmotic pressures are called osmophilic bacteria.
Sudden exposure of bacteria to hypertonic solution may cause osmotic withdrawal of water, leading to shrinkage of the protoplasm (plasmolysis).
On the other hand, sudden transfer of bacteria from concentrated solution to distilled water (hypotonic solutions) may cause excessive entry of water in side the cell leading to swelling and bursting of cell ( plasmoptysis).