MIXED ANAEROBIC INFECTIONS
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MICROBIOLOGY, EPIDEMIOLOGY, AND PATHOGENESIS
Nonsporulating anaerobic bacteria are important components of the normal microbiota of mucosal surfaces of the mouth, lower GI tract, skin, and female genital tract and contribute to physiologic, metabolic, and immunologic functions of the host.
- Most clinically relevant anaerobes are relatively aerotolerant.
- Clinically relevant anaerobes include gram-positive cocci (e.g., Peptostreptococcus spp.), gram-positive rods (e.g., spore-forming clostridia and Propionibacterium acnes), and gram-negative bacilli (e.g., the Bacteroides fragilis group in the intestines, Fusobacterium spp. in the oral cavity and GI tract, Prevotella spp. in the oral cavity and female genital tract, and Porphyromonas spp. in the oral microbiota).
- Infections caused by anaerobes are typically polymicrobial (including at least one anaerobic organism and sometimes involving facultative bacteria) and occur when organisms penetrate a previously sterile site that has a reduced oxidation–reduction potential—e.g., from tissue ischemia, trauma, surgery, perforated viscus, shock, or aspiration. Bacterial synergy, bacterial virulence factors, and mechanisms of abscess formation are factors involved in the pathogenesis of anaerobic infections.
- Anaerobes account for 0.6–0.8% of all cases of bacteremia, with Bacteroides spp. isolated in ∼60% of these cases.