Merck Manual

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Overview of Anaerobic Bacteria


Larry M. Bush

, MD, FACP, Charles E. Schmidt College of Medicine, Florida Atlantic University;

Maria T. Vazquez-Pertejo

, MD, FACP, Wellington Regional Medical Center

Reviewed/Revised Jun 2023

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

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

  • Microaerophilic: Require a low oxygen concentration (typically 2 to 10%) and, for many, a high carbon dioxide concentration (eg, 10%); grow very poorly anaerobically

  • 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:

  • Strict: Tolerate only 0.5% oxygen

  • Moderate: Tolerate 2 to 8% oxygen

  • Aerotolerant anaerobes: Tolerate atmospheric oxygen for a limited time

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 oral cavity (gingival, odontogenic, and pharyngeal), 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

  • Bacteroides (most common): Intra-abdominal infections

  • Fusobacterium: Abscesses, wound infections, and pulmonary and intracranial infections

  • Porphyromonas: Aspiration pneumonia and periodontitis

  • Prevotella: Intra-abdominal, dental, gynecologic, 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 (eg, C. botulinum and C. tetani neurotoxins).

Clues to anaerobic infection include

  • Polymicrobial results on Gram stain or culture

  • Bacteria are seen on Gram stain, but aerobic cultures are sterile

  • 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 oxygen-free atmosphere of carbon dioxide, hydrogen, and nitrogen. Swabs are best transported in an anaerobically sterilized, semisolid medium such as the anaerobic pre-reduced formulation of Cary-Blair transport medium.

NOTE: This is the Professional Version. CONSUMERS: View Consumer Version
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