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Anaerobes comprise the predominant class of bacteria of the normal human microbiota that reside on mucous membranes and predominate in many infectious processes, particularly those arising from mucosal surfaces. These organisms generally cause disease subsequent to the breakdown of mucosal barriers and the leakage of the microbiota into normally sterile sites. Infections resulting from contamination by the microbiota are usually polymicrobial and involve both aerobic and anaerobic bacteria. However, the difficulties encountered in handling specimens in which anaerobes may be important and the technical challenges entailed in cultivating and identifying these organisms in clinical microbiology laboratories continue to leave the anaerobic etiology of an infectious process unproven in many cases. Therefore, an understanding of the types of infections in which anaerobes can play a role is crucial in selecting appropriate microbiologic tools to identify the organisms in clinical specimens and in choosing the most appropriate treatment, including antibiotics and surgical drainage or debridement of the infected site. This chapter focuses on infections caused by anaerobic bacteria other than Clostridium species, which are covered elsewhere (Chaps. 129 and 149).


Anaerobic organisms were first identified by Antonie van Leeuwenhoek in 1680—nearly a century before oxygen itself was discovered. Leeuwenhoek set up culture medium (crushed pepper powder and clean rainwater) in two glass tubes—one open to ambient air and the other sealed closed—that he incubated for several days. Although he did not expect to observe anything in the sealed tube, he was surprised to find “animalcules” in both tubes. He noted that these bacteria in the sealed tube were “bigger than the biggest sort” in the tube left open to air. It was not until the mid- to late nineteenth century that Leeuwenhoek’s findings were confirmed by Pasteur and others. However, these principles described by Leeuwenhoek underlie the basic pathogenesis of anaerobic infections: development of an anaerobic environment in a closed space is due to consumption of oxygen by aerobic organisms and results in the outgrowth of anaerobic organisms.


Anaerobic bacteria can be categorized as obligate anaerobes (killed in the presence of ≥0.5% oxygen), aerotolerant organisms (can tolerate the presence of oxygen but cannot use it for growth), and facultative anaerobes (can grow in the presence or absence of oxygen). Most clinically relevant anaerobes, such as Bacteroides fragilis, Prevotella melaninogenica, and Fusobacterium nucleatum, are relatively aerotolerant. These organisms contrast with obligate aerobes, which require high concentrations of oxygen for growth, and microaerophilic organisms, which are damaged by atmospheric concentrations of oxygen (~21%) but require low concentrations of oxygen (typically 2–10%) for growth. Given that molecular oxygen can reduce to superoxide (O2) and hydrogen peroxide (H2O2), which are damaging to cells, the ability to tolerate the presence of oxygen is due, in part, to the expression of ...

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