Oxygen I Classification of Bacteria Based on Oxygen

Classification of Bacteria Based on Oxygen Requirement:

 

Introduction

Oxygen is a vital factor that influences bacterial metabolism, growth, and survival. Bacteria are classified into different categories based on their oxygen requirements, which depend on the presence or absence of enzymes that neutralize toxic oxygen byproducts such as superoxide radicals and hydrogen peroxide.

 

[1] Obligate Aerobes (Strict Aerobes)

• Oxygen Requirement: Require oxygen for growth and survival.
• Metabolism: Perform aerobic respiration, using oxygen as the final electron acceptor in the electron transport chain (ETC) to generate ATP.
• Enzymes Present:
  • Superoxide dismutase (SOD) – Converts harmful superoxide radicals into oxygen and hydrogen peroxide.
  • Catalase or Peroxidase – Breaks down hydrogen peroxide (H₂O₂) into water and oxygen.
• Habitat: Found in oxygen-rich environments such as soil, water, and the human respiratory system.
• Examples:
  • Mycobacterium tuberculosis (causes tuberculosis)
  • Pseudomonas aeruginosa (found in soil and water)
  • Bacillus subtilis (used in biotechnology)

 

[2] Obligate Anaerobes (Strict Anaerobes)

• Oxygen Requirement: Cannot survive in the presence of oxygen; oxygen is toxic to them.
• Metabolism: Perform anaerobic respiration or fermentation, using alternative electron acceptors (e.g., nitrate, sulfate) instead of oxygen.
• Enzymes Present:
  • Lack superoxide dismutase (SOD) and catalase, making them highly sensitive to oxygen.
• Habitat: Found in deep soil, sediments, intestines, and oxygen-depleted environments.
• Examples:
  • Clostridium botulinum (causes botulism)
  • Clostridium tetani (causes tetanus)
  • Bacteroides fragilis (found in the human gut)

 

[3] Facultative Anaerobes

• Oxygen Requirement: Can survive in both the presence and absence of oxygen.
• Metabolism:
  • Prefer aerobic respiration when oxygen is available (more energy-efficient).
  • Switch to anaerobic respiration or fermentation when oxygen is absent.
• Enzymes Present:
  • Superoxide dismutase (SOD) and catalase allow them to tolerate oxygen.
• Habitat: Found in a variety of environments, including soil, water, food, and the human body.
• Examples:
  • Escherichia coli (commonly found in the intestine)
  • Staphylococcus aureus (opportunistic pathogen)
  • Salmonella typhi (causes typhoid fever)

 

[4] Microaerophiles

• Oxygen Requirement: Require oxygen but at lower concentrations (2-10%) than atmospheric levels (~21%).
• Metabolism:
  • Perform aerobic respiration but are sensitive to high oxygen levels.
• Enzymes Present:
  • Limited amounts of superoxide dismutase (SOD) and catalase, making them susceptible to oxidative stress.
• Habitat: Found in environments with low oxygen, such as deep water, stomach lining, and soil.
• Examples:
  • Helicobacter pylori (causes stomach ulcers)
  • Campylobacter jejuni (causes foodborne illness)

 

[5] Aerotolerant Anaerobes

• Oxygen Requirement: Do not use oxygen but can tolerate its presence.
• Metabolism:
  • Only perform fermentation (anaerobic metabolism), even in the presence of oxygen.
• Enzymes Present:
  • Superoxide dismutase (SOD) present to neutralize toxic oxygen products.
  • Lack catalase, so they cannot break down hydrogen peroxide efficiently.
• Habitat: Found in environments with varying oxygen levels, such as soil, dairy products, and intestines.
• Examples:
  • Lactobacillus acidophilus (used in yogurt and probiotics)
  • Streptococcus pyogenes (causes strep throat)