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Anaerobic bacteria are organisms that require reduced oxygen tension for growth, failing to grow on the surface of solid media in 10% CO 2 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. In general, anaerobes associated with human infections are relatively aerotolerant. . | Chapter 157. Infections Due to Mixed Anaerobic Organisms 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. In general anaerobes associated with human infections are relatively aerotolerant. They can survive for as long as 72 h in the presence of oxygen although 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. The nonsporulating anaerobic bacteria exist as components of the normal flora on the mucosal surfaces of humans and animals. The major reservoirs of these bacteria are the mouth lower gastrointestinal GI tract skin and female genital tract Table 157-1 . Among the constituents of the oral flora anaerobes are the predominant commensal organisms ranging in concentration from 109 mL in saliva to 1012 mL in gingival scrapings. 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. In the colon 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. Table 157-1 Anaerobic
