Bacteria can be classified by their need and tolerance for oxygen:
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Facultative: Grow aerobically or anaerobically in the presence or absence of oxygen
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Microaerophilic: Require a low oxygen concentration (typically 2 to 10%) and, for many, a high carbon dioxide concentration (eg, 10%); grow very poorly anaerobically
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Obligate anaerobic: Are incapable of aerobic metabolism but are variably tolerant of oxygen
Obligate anaerobes replicate at sites with low oxidation-reduction potential (eg, necrotic, devascularized tissue). Oxygen is toxic to them. Obligate anaerobes have been categorized based on their oxygen tolerance:
The obligate anaerobes that commonly cause infection can tolerate atmospheric oxygen for at least 8 hours and frequently for up to 72 hours.
Obligate anaerobes are major components of the normal microflora on mucous membranes, especially of the mouth, lower gastrointestinal (GI) tract, and vagina; these anaerobes cause disease when normal mucosal barriers break down.
Gram-negative anaerobes and some of the infections they cause include
Gram-positive anaerobes and some of the infections they cause include
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Actinomyces: Head, neck, abdominal, and pelvic infections and aspiration pneumonia (actinomycosis)
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Clostridia: Intra-abdominal infections (eg, clostridial necrotizing enteritis), soft-tissue infections, and gas gangrene due to C. perfringens; food poisoning due to C. perfringens type A; botulism and infant botulism due to C. botulinum; tetanus due to C. tetani; and Clostridioides (formerly, Clostridium) difficile–induced diarrhea (pseudomembranous colitis)
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Peptostreptococcus: Oral, respiratory, and intra-abdominal infections
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Propionibacterium: Foreign body infections (eg, in a cerebrospinal fluid shunt, prosthetic joint, or cardiac device)
Anaerobic infections are typically suppurative, causing abscess formation and tissue necrosis and sometimes septic thrombophlebitis, gas formation, or both. Many anaerobes produce tissue-destructive enzymes, as well as some of the most potent paralytic toxins known.
Usually, multiple species of anaerobes are present in infected tissues; aerobes are frequently also present (mixed anaerobic infections).
Clues to anaerobic infection include
Testing
Specimens for anaerobic culture should be obtained by aspiration or biopsy from normally sterile sites. Delivery to the laboratory should be prompt, and transport devices should provide an oxygen-free atmosphere of carbon dioxide, hydrogen, and nitrogen. Swabs are best transported in an anaerobically sterilized, semisolid medium such as Cary-Blair transport medium.
