DEFINITIONS

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 Human Flora: An Overview

Anatomic Site	Total Bacteria a	Aerobic/Anaerobic Ratio	Potential Pathogens
Oral cavity
Saliva	108–109	1:1	Fusobacterium nucleatum, Prevotella melaninogenica, Prevotella oralis group, Bacteroides ureolyticus group, Peptostreptococcus spp.
Tooth surface	1010–1011	1:1
Gingival crevices	1011–1012	103:1
Gastrointestinal tract
Stomach	0–105	1:1
Jejunum/ileum	104–107	1:1
Terminal ileum and colon	1011–1012	103:1	Bacteroides spp. (principally members of the B. fragilis group), Prevotella spp., Clostridium spp., Peptostreptococcus spp.
Female genital tract	107–109	10:1	Peptostreptococcus spp., Bacteroides spp., Prevotella bivia

^a Per gram or milliliter.

Anaerobes play a key role in maintaining the balance between the host and its colonizing organisms. Hundreds of species of anaerobic bacteria have been identified as part of the normal flora of humans. Identification of as many as 500 anaerobic species in fecal specimens reflects the diversity of the anaerobic flora. Despite the complex array of bacteria in the normal flora, relatively few species are isolated commonly from human infection. Anaerobic infections occur when the harmonious relationship between the host and the bacteria is disrupted. Any site in the body is susceptible to infection with these indigenous organisms when a mucosal barrier or the skin is compromised by surgery, trauma, tumor, ischemia, or necrosis, all of which can reduce local tissue redox potentials. Because the sites that are colonized by anaerobes contain many species of bacteria, disruption of anatomic barriers allows the penetration of many organisms, resulting in mixed infections involving multiple species of anaerobes combined with facultative or microaerophilic organisms. Such mixed infections are seen in the head and neck (chronic sinusitis, chronic otitis media, Ludwig’s angina, and periodontal abscesses). Brain abscesses and subdural empyema are the most common anaerobic infections of the central nervous system (CNS). Anaerobes are responsible for pleuropulmonary diseases such as aspiration pneumonia, necrotizing pneumonia, lung abscess, and empyema. These organisms also play an important role in various intraabdominal infections, such as peritonitis and intraabdominal and hepatic abscesses (Chap. 121). They are isolated frequently in female genital tract infections, such as salpingitis, pelvic peritonitis, tuboovarian abscess, vulvovaginal abscess, septic abortion, and endometritis (Chap. 124). Anaerobic bacteria are also found often in infections of the skin, soft tissues, and bones and in bacteremia.

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