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Overview of Anaerobic Bacteria

By

Larry M. Bush

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


Maria T. Vazquez-Pertejo

, MD, FCAP, Wellington Regional Medical Center

Last full review/revision Sep 2019| Content last modified Sep 2019
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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 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

  • 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

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.

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NOTE: This is the Professional Version. CONSUMERS: Click here for the Consumer Version
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