Anaerobic bacteria are organisms that require reduced oxygen tension for growth, failing to grow on the surface of solid media in 10% CO2 in air. (In contrast, microaerophilic bacteria can grow in an atmosphere of 10% CO2 in air or under anaerobic or aerobic conditions, although they grow best in the presence of only a small amount of atmospheric oxygen, and facultative bacteria can grow in the presence or absence of air.) This chapter describes infections caused by nonsporulating anaerobic bacteria. Most clinically relevant anaerobes, such as Bacteroides fragilis, Prevotella melaninogenica, and Fusobacterium nucleatum, are relatively aerotolerant. Although they can survive for sustained periods in the presence of up to 2–8% oxygen, generally they do not multiply in this environment. A far smaller number of pathogenic anaerobic bacteria (which are also part of the normal flora) die after brief contact with oxygen, even in low concentrations.


Most human mucocutaneous surfaces harbor a rich indigenous flora composed of aerobic and anaerobic bacteria. These surfaces are dominated by anaerobic bacteria, which often account for 99.0–99.9% of the culturable flora and range in concentration from 109/mL in saliva to 1012/mL in gingival scrapings and the colon. Most of the normal anaerobic flora cannot be grown or characterized by current laboratory methods. The major reservoirs of these bacteria are the mouth, lower gastrointestinal tract, skin, and female genital tract (Table 164-1). In the oral cavity, the ratio of anaerobic to aerobic bacteria ranges from 1:1 on the surface of a tooth to 1000:1 in the gingival crevices. Anaerobic bacteria are not found in appreciable numbers in the normal upper intestine until the distal ileum. In the colon, the proportion of anaerobes increases significantly, as does the overall bacterial count; for example, there are 1011–1012 organisms per gram of stool, and >99% of these organisms are anaerobic, with an anaerobe-to-aerobe ratio of ∼1000:1. In the female genital tract, there are ∼109 organisms per milliliter of secretions, with an anaerobe-to-aerobe ratio of ∼10:1.

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Table 164-1 Anaerobic Human Flora: An Overview 

Commensal anaerobes have been implicated as crucial mediators of physiologic, metabolic, and immunologic functions of the ...

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