MIXED ANAEROBIC INFECTIONS is a topic covered in the Harrison's Manual of Medicine.

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Microbiology, Epidemiology, and Pathogenesis

Nonsporulating anaerobic bacteria are important components of the normal flora 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 and can survive for prolonged periods in the presence of low levels of oxygen.
    • 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 microaerophilic and 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 0.5–12% of all cases of bacteremia, with B. fragilis isolated in 60–80% of these cases.

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