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Overview of Anaerobic Bacteria

By Joseph R. Lentino, MD, PhD, Chief, Infectious Disease Section and Professor of Medicine, Loyola University Medical Center

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

  • Facultative: Grow aerobically or anaerobically in the presence or absence of O2

  • Microaerophilic: Require a low O2 concentration (eg, 5%) and, for many, a high CO2 concentration (eg, 10%); grow very poorly anaerobically

  • Obligate anaerobic: Are incapable of aerobic metabolism but are variably tolerant of O2

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 O2 tolerance:

  • Strict: Tolerate only 0.5% O2

  • Moderate: Tolerate 2 to 8% O2

  • Aerotolerant anaerobes: Tolerate atmospheric O2 for a limited time

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

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

  • 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


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.