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.

Clinical Manifestations

The clinical presentation of anaerobic infections depends, in part, on the anatomic location affected.

• Mouth, head, and neck infections: Odontogenic infections (e.g., dental caries, periodontal disease, gingivitis) are common, can spread locally, and may be life-threatening.
  • Acute necrotizing ulcerative gingivitis (trench mouth, Vincent's stomatitis) is associated with bleeding tender gums, foul breath, and ulceration with yellowish-white or gray “pseudomembranes.”
  • Noma (cancrum oris) is a gangrenous infection that destroys the soft and hard tissues related to the oral cavity. Noma occurs most frequently in 1- to 4-year-old children with malnutrition or systemic disease, particularly those in sub-Saharan Africa.
  • Peritonsillar abscess (quinsy) is the most common peripharyngeal infection and occurs as a complication of acute tonsillitis, with adolescents most commonly affected. Pts present with sore throat, dysphagia, peritonsillar swelling, muffled voice, and uvular deviation to the contralateral side.
  • Chronic sinusitis and otitis (Chap. 59 Sore Throat, Earache, and Upper Respiratory Symptoms) are commonly due to anaerobes.
  • Complications of anaerobic mouth, head, and neck infections include Lemierre's syndrome, osteomyelitis, CNS infections (e.g., brain abscess, epidural abscess, subdural empyema), mediastinitis, pleuropulmonary infections, and hematogenous dissemination.
    • Lemierre's syndrome, which is typically due to Fusobacterium necrophorum, is an acute oropharyngeal infection with secondary septic thrombophlebitis of the internal jugular vein and frequent septic emboli, most commonly to the lung.
    • Pleuropulmonary infections include aspiration pneumonia (which is difficult to distinguish from chemical pneumonitis due to aspiration of alveolar irritants, such as gastric juices), lung abscesses, and empyema. Antibiotic therapy is not indicated for aspiration pneumonia unless bacterial superinfection occurs. Anaerobic lung abscesses usually arise from a dental source.
• Intraabdominal infections: See Chap. 84 Intraabdominal Infections.
• Pelvic infections: See Chap. 86 Sexually Transmitted and Reproductive Tract Infections for details. Anaerobes, typically in combination with coliforms, are isolated from most women with genital tract infections that are not caused by a sexually transmitted pathogen (e.g., pelvic inflammatory disease, pelvic abscess, endometritis, tubo-ovarian abscess, postoperative or postpartum infections). The major anaerobic pathogens are the B. fragilis group and Prevotella spp. (bivia, disiens), but many other anaerobes have also been implicated.
• Skin and soft-tissue infections: See Chap. 87 Infections of the Skin, Soft Tissues, Joints, and Bones for details. Skin or soft-tissue injury due to trauma, ischemia, or surgery creates a suitable environment for anaerobic infections. The most common locations for anaerobic cellulitis include the neck, trunk, groin, and legs.
• Bone and joint infections: Actinomycosis is the leading cause of anaerobic bone infections worldwide, particularly of the mandible. Anaerobic arthritis is uncommon, typically involves a single isolate, and often results from hematogenous spread; P. acnes, peptostreptococci, and B. fragilis are among the most common anaerobic causes of septic arthritis.