What are the factors that influence bacterial growth

1. Oxygen factors: On basis of their oxygen requirements bacteria are classified as:
  • Obligate aerobes: Growth only in the presence of oxygen (e.g. Pseudomonas, M. tuberculosis, Bacillus, Brucella and Nocardia)
  • Facultative anaerobes: They are aerobes that can also grow anaerobically (e.g. most of the pathogenic bacteria, e.g. E.coli, S.aureus, etc).
  • Facultative aerobes: They are anaerobes that can also growth aerobically (e.g. Lactobacillus)
  • Microaerophilic bacteria: Can growth in the presence of 5–10% of oxygen (e.g. Campylobacter, Helicobacter, Mycobacterium bovis).
  • Obligate anaerobes: Grow only in the absence of oxygen. Oxygen is lethal to these bacteria (e.g. Clostridium)
  • Aerotolerant anaerobe: They can tolerate oxygen for some time, but do not use it (Clostridium histolyticum).

2. Carbon dioxide factors: Capnophilic bacteria need 5–10% of CO2. E.g.Brucella abortus, Streptococcus pneumoniae, etc.

3. Temperature factors: Based on optimal temperature needed for growth, bacteria can be grouped into:

Psychrophiles: Growth below 20°C, e.g. saprophytes. Mesophiles: Growth between 25°C and 40°C, e.g. most of the pathogenic bacteria Thermophiles: Grow above of 55°C - 80°C, e.g. Bacillus stearothermophilus
  • Psychrophiles: Growth below 20°C, e.g. saprophytes.
  • Mesophiles: Growth between 25°C and 40°C, e.g. most of the pathogenic bacteria
  • Thermophiles: Grow above of 55°C - 80°C, e.g. Bacillus stearothermophilus

4. pH factors: Most pathogenic bacteria grow between pH 7.2 - pH 7.6. Very few bacteria (lactobacilli) can grow at acidic pH below pH 4, while bacteria such as Vibrio cholerae are capable of growing at alkaline pH (8.2–8.9).

5. Light factors: Bacteria (except phototrophs) grow well in dark. They are sensitive to ultraviolet rays and other radiations in light. Photochromogenic mycobacteria produce pigments only on exposure to light.

6. Other factors: Osmotic Effect, Mechanical and sonic Stresses factors, Moisture and desiccation.

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