12. Anaerobes

Anaerobic Bacteria Overview

  • Anaerobic bacteria include notable genera such as:

    • Bacteroides

    • Tannerella

    • Porphyromonas

    • Prevotella

    • Fusobacterium

    • Aggregatibacterium

    • Actinomyces

    • Propionibacteria

  • Presentation by Dr. Edit Urbán on February 27, 2025.

Introduction to Anaerobic Organisms

  • Anaerobic Organisms:

    • Do not require oxygen for growth and may die if exposed to it.

  • Aerobic Organisms: Require oxygen to grow.

  • Factors inhibiting Anaerobe Growth:

    • Toxic compounds

    • Lack of enzymes such as catalase and superoxide dismutase

    • Oxidation of essential sulfhydryl groups in enzymes

Types of Anaerobes

  1. Obligate Anaerobes:

    • Can grow without free oxygen but do not multiply in its presence.

    • Examples: Bacteroides fragilis, Clostridium perfringens.

  2. Aerotolerant Anaerobes:

    • Show limited growth in the presence of oxygen but thrive in anaerobic conditions.

    • Examples: C. carnis, C. histolyticum.

  3. Microaerophilic Bacteria:

    • Require low oxygen as a terminal electron acceptor (5% O2) for growth.

    • Example: Campylobacter jejuni.

Energy Metabolism in Anaerobes

  • Fermentation:

    • Some obligate anaerobes (e.g., lactic acid fermentation):

      • Equation: C6H12O6 + 2 ADP + 2 phosphate → 2 lactic acid + 2 CO2 + 2 ATP.

      • Releases approximately 150 kJ per mol.

  • Alcohol Fermentation:

    • Also utilized by anaerobes in low oxygen:

      • Equation: C6H12O6 → 2 C2H5OH + 2 CO2 + 2 ATP.

      • Releases about 180 kJ per mol.

  • Other fermentative pathways include propionic acid and butyric acid fermentation.

Classification of Anaerobic Bacteria

  1. Gram-negative Bacilli:

    • Includes Bacteroides, Borrelia, Fusobacterium.

  2. Gram-positive Cocci:

    • Examples: Anaerococcus, Peptococcus.

  3. Non-sporing Gram-positive Bacilli:

    • Examples: Actinomyces, Lactobacillus.

  4. Gram-negative Cocci:

    • Examples: Acidaminococcus, Veillonella.

Laboratory Identification

  • High morbidity and mortality associated with these bacteria.

  • Common Isolates:

    • Primarily from clinical specimens: Bacteroides fragilis, Prevotella, Porphyromonas, Fusobacterium.

  • Resistance to common antibiotics, notably penicillin.

Infections Caused by Anaerobes

  • Types of infections include:

    • Abscesses, pneumonia, endocarditis, and various dental infections.

  • Commonly associated with conditions like appendicitis, cholecystitis, and aspiration pneumonia.

Virulence Factors of Anaerobes

  • Include capsules, toxins, and enzymes such as collagenase and heparinase. These help evade host defenses and facilitate tissue damage.

  • Specific species like Bacteroides fragilis and Propionibacterium also exhibit unique virulence mechanisms.

Clinical Significance

  • Anaerobes contribute significantly to polymicrobial infections, often arising from indigenous microbiota.

  • Infections occur when mucosal barriers are breached, such as through trauma or disease.

  • Gram-negative anaerobes can enter circulation, leading to serious systemic infections like bacteremia.

Epidemiology and Transmission

  • Anaerobic bacteria are normal inhabitants of the oral cavity and gastrointestinal tract.

  • Bacteroides species are particularly important in human infections and can establish in the gut from infancy.

Conclusion

  • Proper identification and treatment of anaerobic infections are critical due to their polymicrobial nature and potential for severe outcomes.

  • Ongoing awareness of their pathogenic potential, especially in immunocompromised individuals, is essential.