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.
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 gray exudates.
- - Noma (cancrum oris) is a necrotizing infection of the oral mucous membranes that rapidly evolves from gingival inflammation to orofacial gangrene with destruction of bone and soft tissue. Noma occurs most frequently in 1- to 4-year-old children with malnutrition or systemic disease, particularly those in sub-Saharan Africa.
- - Acute necrotizing infection of the pharynx is associated with ulcerative gingivitis. Pts have a sore throat, foul breath, fever, a choking sensation, and tonsillar pillars that are swollen, red, ulcerated, and covered with a gray membrane. Aspiration of infected material can lead to a lung abscess.
- - Peripharyngeal infections include peritonsillar abscess (quinsy; caused by a mixed flora including anaerobes and group A streptococci) and submandibular space infection (Ludwig's angina), which arises from the second and third molars in 80% of cases and is associated with swelling (sometimes leading to respiratory obstruction), pain, trismus, and displacement of the tongue.
- - Chronic sinusitis and otitis (Chap. 58. 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 metastasis, most commonly to the lung. A typical clinical triad includes pharyngitis, a tender/swollen neck, and noncavitating pulmonary infiltrates.
- Pleuropulmonary infections include aspiration pneumonia (which is difficult to distinguish from chemical pneumonitis due to aspiration of gastric juices), necrotizing pneumonitis, lung abscesses, and empyema. Bacterial aspiration pneumonia is associated with a depressed gag reflex, impaired swallowing, or altered mental status; anaerobic lung abscess usually arises from a dental source.
- Intraabdominal infections: See Chap. 81. Intraabdominal Infections.
- Pelvic infections: See Chap. 83. Sexually Transmitted and Reproductive Tract Infections for more details. Anaerobes, typically in combination with coliforms, are isolated from most women with genital tract infections (e.g., pelvic inflammatory disease, pelvic abscess, endometritis, tuboovarian abscess, postoperative or postpartum infections) that are not caused by a sexually transmitted pathogen. The major anaerobic pathogens are B. fragilis, Prevotella spp. (bivia, disiens, melaninogenica), anaerobic cocci, and Clostridium spp.
- Skin and soft tissue infections: See Chap. 84. Infections of the Skin, Soft Tissues, Joints, and Bones for more details. These infections most frequently occur at sites prone to contamination with feces or with upper airway secretions.
- Bone and joint infections: Anaerobic bone and joint infections usually occur adjacent to soft tissue infections. Actinomycosis is the leading cause of anaerobic bone infections worldwide; Fusobacterium spp. are the most common anaerobic cause of septic arthritis.
The three critical steps in successfully culturing anaerobic bacteria from clinical samples are (1) proper specimen collection, with avoidance of contamination by the normal microbiota; (2) rapid specimen transport to the microbiology laboratory in anaerobic transport media; and (3) proper specimen handling. A foul odor is often indicative (and nearly pathognomonic) of an anaerobic infection.
Treatment: Mixed Anaerobic Infections
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For a more detailed discussion, see Thwaites CL, Yen LM: Tetanus, Chap. 177, p. 984; Maslanka S, Rao AK: Botulism, Chap. 178, p. 987; Bryant AE, Stevens DL: Gas Gangrene and Other Clostridial Infections, Chap. 179, p. 990; and Cohen-Poradosu R, Kasper DL: Infections Due to Mixed Anaerobic Organisms, Chap. 201, p. 1094, in HPIM-19. |
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