Environmental Controls on Microbial Growth

Environmental Limits on Microbial Growth

• Temperature
• Pressure
• Osmolarity
• Oxygen
• pH

Factors Affecting Bacterial Growth Rates

Temperature

• Cardinal temperatures are crucial for microbial growth:
  • Minimum temperature: below this level, no growth occurs.
  • Optimum temperature: point of most rapid growth.
  • Maximum temperature: above this, no growth occurs.
• Cardinal temperatures vary among different organisms.

Temperature Classes of Microorganisms

• Psychrophiles

  • Optimum temperature: <15°C
  • Minimum temperature: < 0°C
  • Maximum temperature: < 20°C
  • Habitat: Polar regions and under ice, e.g., Chlamydomonas nivalis.

• Psychrotolerant

  • Optimum temperature: < 20-40°C
  • Minimum temperature: < 0°C
  • Maximum temperature: < 45°C
  • Habitat: Refrigerated foods, includes various bacteria, fungi, and algae.

• Mesophiles

  • Optimum temperature: < 39°C
  • Minimum temperature: < 10°C
  • Maximum temperature: < 48°C
  • Habitat: Microbial flora of mammals, e.g., E. coli.

• Thermophiles

  • Optimum temperature: > 45°C
  • Minimum temperature: < 40°C
  • Maximum temperature: < 68°C
  • Habitat: Extreme environments like hot springs and oceanic thermal vents.

• Hyperthermophiles

  • Optimum temperature: > 80°C
  • Minimum temperature: < 65°C
  • Maximum temperature: > 100°C
  • Habitat: Hot springs, oceanic thermal vents.

Response to Temperature

• Heat shock response:
  • Triggered at high temperature ranges, producing emergency proteins that help maintain protein structure.
  • Induced by stressors like heat, high salt concentrations, and arid conditions.

Variations in Pressure

• Barophiles: Adapted to high-pressure environments (up to 1,000 atm).
• Barotolerant organisms: Can grow under high pressure but not extreme pressures.
• Barosensitive organisms: Do not survive under high pressure; includes most typical bacteria and mammals.

Changes in Water Activity

• Halophiles: Require high concentrations of NaCl (2–4 M, 10x seawater), live in saline environments.

Osmolarity

• Increased solute concentrations raise osmolarity, reducing available water.
• High osmolarity can burst cell membranes due to osmotic pressure.
• Cells adapt by synthesizing osmolytes to increase internal osmolarity.

Effect of pH

• Most bacteria thrive at neutral pH; enzymes function within a narrow pH range.
• Amino acid charges are affected by pH levels, influencing bacterial growth.
• Good food preservatives often exploit weak acids that can disrupt cellular pH homeostasis.
• Acidophilic bacteria: Can thrive in very low pH environments, e.g., Helicobacter pylori in the stomach.
• Alkaliphilic bacteria: Preferring basic environments like soda lakes, e.g., Bacillus firmus.

pH Scale

• 0: Battery acid
• 1: Strong acids
• 5: Soft drinks
• 7: Pure water
• 14: Drain cleaners

Oxygen

• Obligate aerobes: Require O2 for growth, e.g., Micrococcus luteus.
• Obligate anaerobes: Cannot grow in the presence of O2, e.g., Methanobacterium formicicum.
• Aerotolerant anaerobes: Do not utilize O2 but can grow in its presence, e.g., Streptococcus pyogenes.
• Microaerophiles: Thrive in low O2 environments, e.g., Spirillum volutans.
• Facultative anaerobes: Can grow without O2 but utilize it when available, e.g., Escherichia coli.