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Medically important infections caused by anaerobic bacteria are common. The infections are often polymicrobial—that is, the anaerobic bacteria are found in mixed infections with other anaerobes, facultative anaerobes, and aerobes (see the glossary of definitions). Anaerobic bacteria are found throughout the human body—on the skin, on mucosal surfaces, and in high concentrations in the mouth and in gastrointestinal tract—as part of the normal microbiota (see Chapter 10). Infection results when anaerobes and other bacteria of the normal microbiota contaminate normally sterile body sites.

Several important diseases are caused by anaerobic Clostridium species from the environment or from normal flora: botulism, tetanus, gas gangrene, food poisoning, and pseudomembranous colitis. These diseases are discussed in Chapters 9 and 11.


Aerobic bacteria: Bacteria that require oxygen as a terminal electron acceptor and will not grow under anaerobic conditions (ie, in the absence of O2). Some Bacillus species and Mycobacterium tuberculosis are obligate aerobes (ie, they must have oxygen to survive).

Anaerobic bacteria: Bacteria that do not use oxygen for growth and metabolism but obtain their energy from fermentation reactions. A functional definition of anaerobes is that they require reduced oxygen tension for growth and fail to grow on the surface of solid medium in 10% CO2 in ambient air. Bacteroides and Clostridium species are the examples of anaerobes.

Facultative anaerobes: Bacteria that can grow either oxidatively, using oxygen as a terminal electron acceptor, or anaerobically, using fermentation reactions to obtain energy. Such bacteria are common pathogens. Streptococcus species and the Enterobacteriaceae (eg, Escherichia coli) are among the many facultative anaerobes that cause disease. Often, bacteria that are facultative anaerobes are called “aerobes.”


Anaerobic bacteria do not grow in the presence of oxygen and are killed by oxygen or toxic oxygen radicals (see later discussion); pH and oxidation–reduction potential (Eh) are also important in establishing conditions that favor growth of anaerobes. Anaerobes grow at a low or negative Eh.

Aerobes and facultative anaerobes often have the metabolic systems listed below, but anaerobic bacteria frequently do not.

  1. Cytochrome systems for the metabolism of O2

  2. Superoxide dismutase (SOD), which catalyzes the following reaction:

    O-2 + O-2 + 2H+ → H2O2 + O2

  3. Catalase, which catalyzes the following reaction:

    2H2O2 → 2H2O + O2 (gas bubbles)

Anaerobic bacteria do not have cytochrome systems for oxygen metabolism. Less fastidious anaerobes may have low levels of SOD and may or may not have catalase. Most bacteria of the Bacteroides fragilis group have small amounts of both catalase and SOD. There appear to be multiple mechanisms for oxygen toxicity. Presumably, when anaerobes have SOD or catalase (or both), they are able to negate the ...

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