anaerobe
Summary
An anaerobe is a microorganism that does not require oxygen for growth and, in the case of obligate anaerobes, is often harmed by oxygen due to lack of enzymes like catalase and superoxide dismutase (SOD) that detoxify reactive oxygen species. Key anaerobic pathogens include Clostridium, Bacteroides, Fusobacterium, Actinomyces, and Peptostreptococcus. They are clinically important in abscesses, aspiration pneumonia, and deep tissue infections with foul-smelling discharge.
Detail
Anaerobes are classified as obligate anaerobes (cannot survive in oxygen, e.g., Clostridium, Bacteroides), facultative anaerobes (can grow with or without oxygen, e.g., E. coli, staphylococci), and aerotolerant anaerobes (survive but don't use oxygen). Obligate anaerobes lack catalase and/or superoxide dismutase, enzymes needed to neutralize toxic oxygen radicals (superoxide, hydrogen peroxide) generated during aerobic metabolism, making them susceptible to oxygen toxicity.
Clinical significance: Anaerobic infections classically occur in areas with poor blood supply/low oxygen tension - abscesses, deep wounds, aspiration pneumonia, and intra-abdominal infections. Common anaerobic pathogens: Bacteroides fragilis (bowel flora, intra-abdominal abscesses), Clostridium species (C. perfringens - gas gangrene, C. difficile - pseudomembranous colitis, C. tetani - tetanus, C. botulinum - botulism), Fusobacterium (Lemierre syndrome), Actinomyces (sulfur granules, sinus tract formation), and Peptostreptococcus.
Clinical clues suggesting anaerobic infection: foul-smelling discharge, gas production in tissue (crepitus), infection near mucosal surfaces (GI/oral flora), abscess formation, and failure to grow on routine aerobic cultures. Treatment often requires anaerobic-specific antibiotics such as metronidazole, clindamycin, or beta-lactam/beta-lactamase inhibitors, plus surgical drainage or debridement when abscesses or necrotic tissue are present. Culturing anaerobes requires special techniques (anaerobic chambers, thioglycolate broth) since exposure to atmospheric oxygen kills obligate anaerobes.
Sources
- First Aid for the USMLE Step 1
- Sherris Medical Microbiology
- Harrison's Principles of Internal Medicine
- Murray's Medical Microbiology
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