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Anaerobes
can infect normal hosts and those with compromised resistance or
damaged tissues. Symptoms depend on site of infection. Anaerobes
are often accompanied by aerobic organisms. Diagnosis is clinical
combined with Gram stain and anaerobic cultures. Treatment is with
antibiotics and surgical drainage and debridement.
Hundreds of species of nonsporulating anaerobes are part of the normal flora of the skin, mouth, GI tract, and vagina. If this commensal relationship is disrupted (eg, by surgery or other trauma, poor blood supply, tissue necrosis), a few of these species can cause infections with high morbidity and mortality. After becoming established in a primary site, organisms can spread hematogenously to distant sites. Because aerobic and anaerobic bacteria frequently are found in the same infected site, appropriate procedures for isolation and culture are necessary to keep from overlooking the anaerobes. Anaerobes can be the major cause of infection in the pleural spaces and lungs; in intra-abdominal, gynecologic, CNS, upper respiratory tract, and cutaneous diseases; and in bacteremia.
Etiology
and Pathophysiology
The principal anaerobic gram-positive cocci that produce disease are the peptococci and the peptostreptococci, which are part of the normal flora of the mouth, upper respiratory tract, and large intestine. The principal anaerobic gram-negative bacilli include Bacteroides
fragilis
, Prevotella melaninogenica, and Fusobacterium sp. The B. fragilis group is part of the normal bowel flora and includes the anaerobic pathogens most frequently isolated from intra-abdominal infections. Organisms in the Prevotella group and Fusobacterium sp are part of the indigenous oral flora.
Anaerobic infections can usually be characterized by the following features:
Some anaerobic bacteria possess distinct virulence factors. Those of B. fragilis probably account for its frequent isolation from clinical specimens despite its relative rarity in normal flora. This organism has a polysaccharide capsule that apparently stimulates abscess formation. An experimental model of intra-abdominal sepsis has shown that B. fragilis alone can cause abscesses, whereas other Bacteroides sp require the synergistic effect of another organism. Another virulence factor, a potent endotoxin, is implicated in septic shock associated with severe Fusobacterium pharyngitis.
Morbidity and mortality are as great from anaerobic and mixed bacterial sepsis as from sepsis caused by a single aerobic organism. Anaerobic infections are often complicated by deep-seated tissue necrosis. The overall mortality rate for severe intra-abdominal sepsis and mixed anaerobic pneumonias tends to be high. B. fragilis bacteremia has high mortality, especially in the elderly and in patients with cancer.
Symptoms and Signs
Patients usually have fever, rigors, and critical illness; shock may develop. DIC may occur in Fusobacterium sepsis.
Clinical clues to the presence of anaerobic organisms include infection adjacent to mucosal surfaces bearing anaerobic flora; ischemia, tumor, penetrating trauma, foreign body, or perforated viscus; spreading gangrene involving skin, subcutaneous tissue, fascia, and muscle; feculent odor in pus or infected tissues; abscess formation; gas in tissues; septic thrombophlebitis; and failure to respond to antibiotics that do not have significant anaerobic activity.
Specific infections caused by mixed anaerobic organisms are discussed elsewhere in The Manual. A listing of these conditions appears in Table 2: Anaerobic Bacteria: Conditions Often Caused by Mixed* Anaerobic Organisms . Anaerobes are rare in UTI, septic arthritis, and infective endocarditis.
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Table 2
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Conditions Often Caused
by Mixed* Anaerobic Organisms
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Anaerobic cellulitis
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Aspiration pneumonia
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Bartholin's gland infections
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Brain abscesses
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Chronic otitis media
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Chronic sinusitis
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Decubitus or ischemic ulcer infections
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Dental abscesses
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Endometritis
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Epidural and subdural empyema
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Human bite infections
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Intra-abdominal abscess
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Liver abscess
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Ludwig's angina
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Lung abscess
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Mandibular osteomyelitis
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Necrotizing gingivitis
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Necrotizing ulcerative mucositis (cancrum oris)
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Nongonococcal tubo-ovarian abscess
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Parametrial abscess
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Pelvic peritonitis
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Periodontitis
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Peritonitis
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Septic thrombophlebitis
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Skene's glands infection
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Vincent's angina
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*With aerobes or other anaerobes.
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Diagnosis
Anaerobic infection should be suspected in any foul-smelling wound or when a Gram stain of pus from an infected site shows mixed pleomorphic bacteria. Only specimens from normally sterile sites should be cultured, because commensal contaminants may easily be mistaken for pathogens.
Gram stains and aerobic cultures should be obtained for all specimens. Gram stain, particularly in Bacteroides infection, and cultures, for all anaerobes, may be falsely negative. Antibiotic sensitivity testing of anaerobes is exacting, and data may not be available for ≥ 1 wk after initial culture. However, if the species is known, sensitivity patterns usually can be predicted. Therefore, many laboratories do not routinely test anaerobic organisms for sensitivity.
Treatment
and Prevention
In established infection, pus is drained and devitalized tissue, foreign bodies, and necrotic tissue removed. Organ perforations must be closed or drained. Whenever possible, blood supply should be reestablished. Septic thrombophlebitis may require vein ligation as well as antimicrobial therapy.
Because anaerobic culture results may not be available for 3 to 5 days, antibiotics are started. Antibiotics sometimes work even when some of the bacterial species in a mixed infection are resistant to the antibiotic, especially if surgical debridement and drainage are adequate.
Oropharyngeal anaerobic infections should be treated with penicillin G . Infrequently, oral anaerobic infections fail to respond and require a drug effective against penicillin-resistant anaerobes (see below). Lung abscesses should be treated with clindamycin or a β-lactam/β-lactamase combination. In patients allergic to penicillin, clindamycin or metronidazole (plus an agent active against aerobes) is useful.
GI or female pelvic anaerobic infections, which likely contain B. fragilis, may be penicillin resistant. Resistance to 2nd-generation cephalosporins and clindamycin also occurs. No single regimen has been shown to be superior. The following drugs have excellent in vitro activity and are effective: metronidazole , imipenem-cilastatin , piperacillin-tazobactam , ampicillin-sulbactam , meropenem , and ticarcillin -clavulanic acid. All except metronidazole can be used as monotherapy because these drugs also have good activity against aerobes. Drugs that are somewhat less active in vitro but are usually effective include clindamycin , cefoxitin , and cefotetan . Metronidazole 500 to 750 mg IV q 8 h (for children, 10 mg/kg q 8 h) given with an aminoglycoside (eg, gentamicin 5 mg/kg once/day or 2 mg/kg q 8 h; to cover enteric gram-negative flora) can be used for intra-abdominal infection or any infection arising from a colonic source. Clindamycin 900 mg IV q 8 h (for children, 10 mg/kg q 8 h) is an alternative. Metronidazole is active against clindamycin -resistant B. fragilis, has unique anaerobic bactericidal activity, and usually avoids the pseudomembranous colitis sometimes associated with clindamycin . Concerns about its potential mutagenicity have not been of clinical consequence.
Patients undergoing elective colonic surgery should undergo bowel preparation (eg, cathartics, enemas, and oral neomycin and erythromycin ). Preoperative parenteral antibiotics control bacteremia, reduce secondary or metastatic suppurative complications, and prevent local spread of infection around the surgical site. Cefoxitin or a combination of either metronidazole or clindamycin with gentamicin or tobramycin may be used.
Last full review/revision November 2005
Content last modified November 2005
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