The Requirements For Growth

The growth of a population is an increase in the number of cells.

The requirements for microbial growth are both physical and chemical.

Physical Requirements

On the basis of preferred temperature ranges, microbes are classified as psychrophiles (cold-living), mesophiles (moderate-temperature-loving), and thermophiles (heat-loving).

• Psychrophiles can grow at 0° C but optimum is about 15° C.
• Psychrotrophs can grow at 0° C also but optimum is 20 - 30° C – important in food spoilage.
• Mesophiles grow best at moderate around 37° C – many pathogens fall in this category.
• Thermophiles have a growth optimum at around 60° C.
• Hyperthermophiles have growth optima of 80° C or higher (archaea).

The minimum growth temperature is the lowest temperature at which a species will grow, the optimum growth temperature is the temperature at which it grows best, and the maximum growth temperature is the highest temperature at which growth is possible.

Most bacteria grow best at pH value between 6.5 and 7.5.

• Very few grow below a pH of 4, although acidophiles may grow in pH conditions as low as 1.

In a hypertonic solution, most microbes undergo plasmolysis; halophiles can tolerate high salt concentrations and may be obligate or facultative.

All organisms require a carbon source; chemoheterotrophs use an organic molecule and autotrophs typically use carbon dioxide.

Nitrogen is needed for protein and nucleic acid synthesis. Nitrogen can be obtained from the decomposition of proteins or from NH₄⁺ or NO₃^-; a few bacteria are capable of nitrogen (N₂) fixation.

On the basis of oxygen requirements, organisms are classified as obligate aerobes, facultative anaerobes, obligate anaerobes, aerotolerant anaerobes, and microaerophiles.

• Facultative anaerobes can grow in aerobic or anaerobic conditions although tend to grow better in aerobic conditions.
  • Includes E. coli and some yeasts.
• Aerotolerant anaerobes don’t use O₂ but tolerate it – have SOD or some similar enzyme.
  • Many ferment carbohydrate to lactate, which inhibits growth of aerobes.
  • Microaerophilic aerobes require low O₂ concentrations; they are sensitive to oxygen-derived free radicals.

Aerobes, facultative anaerobes, and aerotolerant anaerobes must have the enzymes superoxide dismutase (2 O₂ ^-. + 2 H⁺---> O₂ + H₂O₂ ) and either catalase (2 H₂O₂ ---> 2 H₂O + O₂ ) or peroxidase (H₂O₂ + 2 H⁺---> 2 H₂O).

Other chemicals required for microbial growth include sulfur, phosphorus, trace elements, and, for some microorganisms, organic growth factors.