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Anaerobic bacteria are intolerant of O₂, replicating at low oxidation-reduction potential sites, such as necrotic, devascularized tissue. Microaerophilic organisms are tolerant of low O₂ concentrations but grow better anaerobically or with > 10% CO₂ in the air. Anaerobic organisms can be classified as strict or moderate (eg, intolerant of 0.5% O₂ vs tolerant of 2 to 8% O₂). Facultative anaerobes tolerate O₂ but grow in its absence as well. In humans, anaerobic organisms are among the normal flora (especially of the GI tract, mouth, and vagina), but when they enter sterile spaces, they can cause serious infections.

Anaerobic infections are typically suppurative, causing abscess formation and tissue necrosis, often the result of thrombophlebitis and/or gas formation. Many anaerobes produce enzymes that devitalize tissue as well as some of the most potent paralytic toxins known. Clues to the presence of anaerobic infection include gas formation in tissue, foul feculent odors, and abscess formation or tissue necrosis.

Specimens for anaerobic culture should be obtained as aspirates or biopsy material from normally sterile sites. Delivery to the laboratory should be prompt, and transport devices should provide an O₂-free atmosphere of carbon dioxide, hydrogen, and nitrogen. Swabs are best transported in an anaerobically sterilized, semisolid medium such as Cary-Blair transport medium.

Clostridia: The most notorious of the anaerobic pathogens are the clostridia—spore-forming, gram-positive bacilli found widely in dust, soil, and vegetation and as normal flora in mammalian GI tracts. Although nearly 100 Clostridium sp have been identified, only 25 to 30 commonly induce human or animal disease. The pathogenic species produce tissue-destructive and neural exotoxins that are responsible for disease manifestations. Clostridia may become pathogenic when tissue O₂ tension and pH are low. Such an anaerobic environment may develop in ischemic or devitalized tissue, as occurs with primary arterial insufficiency, or after severe penetrating or crushing injuries. The deeper and more severe the wound, the more prone the patient is to clostridial infection, especially if there is even minimal contamination by foreign matter. Clostridial disease can also occur after injection of street drugs. Serious noninfectious disease can occur after ingestion of home-canned foods in which clostridia have produced toxins.

Selected Conditions Associated With Clostridial Infections Condition Agent Toxin Soft-tissue infection: crepitant cellulitis, myositis, clostridial myonecrosis C. perfringens α-Toxin (others) Hemolysis C. perfringens Phospholipase C     α-Toxin Muscle necrosis C. perfringens θ-Toxin Enteric diseases     Food poisoning C. perfringens type A Enterotoxin Enteritis necroticans C. perfringens type C β-Toxin Antibiotic-associated colitis C. difficile Toxin A Neutropenic enterocolitis C. septicum (others) Unknown, possibly β-toxin Colorectal malignancy C. septicum   Hemolysis by septicolysine C. septicum δ-Toxin Tissue necrosis C. septicum α-Toxin DNA lysis by DNase C. septicum β-Toxin Hyaluronan lysis by hyaluronilase C. septicum γ-Toxin Neurologic syndromes     Tetanus C. tetani Tetanospasmin Botulism C. botulinum Botulinal toxins A-G Abdominal infections: Cholecystitis, peritonitis, ruptured appendix, bowel perforation, neutropenic enterocolitis C. perfringens , C. ramosum (many others) β-Toxin

The most frequent clostridial infection is minor, self-limited gastroenteritis typically from C. perfringens type A. Serious clostridial diseases are relatively rare but can be fatal. Enteritis necroticans is caused by C. perfringens type C, antibiotic-associated colitis by C. difficile, and neutropenic enterocolitis by C. septicum. Abdominal disorders, such as cholecystitis, peritonitis, ruptured appendix, and bowel perforation can involve C. perfringens , C. ramosum, and many others. Muscle necrosis and soft-tissue infection, which is characterized by crepitant cellulitis, myositis, and clostridial myonecrosis, can be caused by C. perfringens. Tissue necrosis can be caused by C. septicum. Tetanus is caused by C. tetani, botulism by C. botulinum. Clostridia also appear as components of mixed flora in common mild wound infections; their role in such infections is unclear.

Hospital-acquired clostridial infection is increasing, particularly in postoperative and immunocompromised patients. Severe clostridial sepsis may complicate intestinal perforation and obstruction.

Other anaerobes: Organisms of concern include Actinomyces israelii, a cause of chronic localized or hematogenous infection, and a host of nonsporulating anaerobes, both cocci and bacilli, most of which are commensals until they invade normally sterile spaces.

Last full review/revision November 2005

Content last modified February 2008