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.
Table 164-1 Anaerobic Human Flora: An Overview
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Table 164-1 Anaerobic Human Flora: An Overview
|Anatomic Site||Total Bacteriaa||Anaerobic/Aerobic Ratio||Potential Pathogens|
|Fusobacterium nucleatum, Prevotella melaninogenica, Prevotella oralis group, Bacteroides ureolyticus group, Peptostreptococcus spp.|
Terminal ileum and colon
|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|
Commensal anaerobes have been implicated as crucial mediators of physiologic, metabolic, and immunologic functions of the mammalian host. One of the most important roles that anaerobes serve as components of the normal colonic flora is colonization resistance, in which their presence effectively interferes with colonization by potentially pathogenic bacterial species through the depletion of oxygen and nutrients, the production of enzymes and toxic end products, and the modulation of the host's intestinal innate immune response. Bacteroides and other intestinal bacteria ferment carbohydrates and produce volatile fatty acids that are reabsorbed and used by the host as an energy source. The anaerobic intestinal microflora is also responsible for the production of secreted products that promote human health (e.g., vitamin K and bile acids).
The anaerobic intestinal flora influences the development of an intact mucosa and of mucosa-associated lymphoid tissue. Colonization of germ-free mice with a single species, Bacteroides thetaiotaomicron, affects the expression of various host genes and corrects deficiencies of nutrient uptake, metabolism, angiogenesis, mucosal barrier function, and enteric nervous system development. The symbiosis factor polysaccharide A of B. fragilis influences the normal development and function of the mammalian immune system and protects mice against colitis in a model of inflammatory bowel disease.
Hundreds of species of anaerobic bacteria have been identified as part of the normal flora of humans. 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. 127). They are isolated frequently in female genital tract infections, such as salpingitis, pelvic peritonitis, tuboovarian abscess, vulvovaginal abscess, septic abortion, and endometritis (Chap. 130). Anaerobic bacteria are also found often in bacteremia and in infections of the skin, soft tissues, and bones.
The taxonomic classification of anaerobes is rapidly evolving, with frequent changes in nomenclature based on newly discovered relationships among bacterial species. Infections caused by anaerobic bacteria most frequently are due to more than one organism. These polymicrobial infections may be caused by one or several anaerobic species or by a combination of ...