Bacteria can be classified by their need and tolerance for O₂:

• Facultative bacteria, which grow in the presence or absence of O₂
• Microaerophilic bacteria, which tolerate low O₂ concentrations but grow better anaerobically or with > 10% CO₂
• Obligate anaerobic bacteria, which are intolerant of O₂

Obligate anaerobes replicate at sites with low oxidation-reduction potential (eg, necrotic, devascularized tissue). Obligate anaerobes have been categorized based on their O₂ tolerance: strict anaerobes grow in ≤ 0.4% O₂; moderate anaerobes grow in 0.8 to 2.5% O₂; and aerotolerant anaerobes grow in ≥ 2.5% O₂. The obligate anaerobes that commonly cause infection can tolerate atmospheric O₂ for at least 8 h and frequently for up to 72 h.

Obligate anaerobes are major components of the normal microflora on mucous membranes, especially of the mouth, lower 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

• Bacteroides (most common): Intra-abdominal infections
• Fusobacterium: Abscesses, wound infections, and pulmonary and intracranial infections
• Porphyromonas: Aspiration pneumonia and periodontitis
• Prevotella: Intra-abdominal and soft-tissue infections

Gram-positive anaerobes and some of the infections they cause include

• Actinomyces: Head, neck, abdominal, and pelvic infections and aspiration pneumonia
• Clostridium: Gas gangrene due to C. perfringens,, food poisoning due to C. perfringens type A, botulism due to C. botulinum, tetanus due to C. tetani, and C. difficile–induced diarrhea (pseudomembranous colitis)
• Peptostreptococcus: Oral, respiratory, and intra-abdominal infections
• 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 (often the result of thrombophlebitis, gas formation, or both). Many anaerobes produce tissue-destructive enzymes as well as some of the most potent paralytic toxins known.

Clues to anaerobic infection include

• Polymicrobial results on Gram stain or culture
• Gas in pus or infected tissues
• Foul odor of pus or infected tissues
• Necrotic infected tissues
• Site of infection near mucosa where anaerobic microflora normally reside

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 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.

Last full review/revision August 2009 by Joseph R. Lentino, MD, PhD