Factors Affecting Growth of Bacteria
A variety of factors affect growth of bacteria. These are discussed below:
Bacteria on the basis of their oxygen requirements can be classi-fied broadly into aerobic and anaerobic bacteria.
Aerobic bacteria: They require oxygen for their growth. Theymay be:
o Obligate aerobes—which can grow only in the presence of oxygen (e.g., P. aeruginosa).
o Facultative aerobes—which are ordinary aerobes but can also grow without oxygen (e.g., E. coli). Most of the pathogenic bacteria are facultative aerobes.
o Microaerophilic bacteria—those bacteria that 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 (O22) are not produced.
Anaerobic bacteria: Obligate anaerobes are the bacteria thatcan grow only in the absence of oxygen (e.g., Clostridium botulinumClostridium tetani, etc.). These bacteria lack superoxide dismutaseand catalase; hence oxygen is lethal to these organisms.
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 candle jar, 3% CO2 can be achieved. Examples of such bacteria include H. influenzae, Brucellaabortus, etc.
The optimum temperature for most of the pathogenic bacteria is 378C. The optimal temperature, however, is variable; depend-ing on their temperature range, growth of bacteria is grouped as follows:
o Psychrophiles:These bacteria are cold loving microbes thatgrow within a temperature range of 0–208C. Most of soil and water saprophytes belong to this group.
o Mesophiles:These are moderate temperature loving microbesthat grow between 258C and 408C. Most of pathogenic bac-teriabelong to this group.
o Thermophiles:These are heat loving microbes. They can growat a high temperature range of 55–808C. B. stearothermophilus is an example.
Most pathogenic bacteria grow between pH 7.2 and 7.6. Very few bacteria, such as lactobacilli, can grow at acidic pH below 4.0. Many food items, such as pickles and cheese, are prevented from spoilage by acids produced during fermentation.
V. choleraeis an example of the bacteria that can grow at analkaline (8.2–8.9) pH.
Depending on the source of energy bacteria make use of, they may be classified as phototrophs (bacteria deriving energy from sunlight) or chemotrophs (bacteria deriving energy from chem-ical sources).
Microbes obtain almost all their nutrients in solution from surrounding water. Hence factors such as osmotic pressure and salt concentration of the solution affect the growth of bacteria. Bacteria by virtue of mechanical strength of their cell wall are able to withstand a wide range of external osmotic variations. Organisms requiring high osmotic pressures are called osmo-philic bacteria. Sudden exposure of bacteria to hypertonic solu-tion may cause osmotic withdrawal of water, leading to osmotic shrinkage of the protoplasm ( plasmolysis). On the other hand, sudden transfer of bacteria from concentrated solution to dis-tilled water may cause excessive imbibition of water leading to swelling and bursting of cell ( plasmoptysis).