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For further information, see CMDT Part 33-31: Anaerobic Infections

Key Features

  • Each gram of stool contains up to 1011 anaerobes, predominantly

    • Bacteroides fragilis

    • Clostridia

    • Peptostreptococci

  • These organisms play a central role in most intra-abdominal abscesses after trauma to the colon and in

    • Diverticulitis

    • Appendicitis

    • Perirectal abscess

  • They may also participate in hepatic abscess and cholecystitis

  • The bacteriology of these infections includes anaerobes as well as enteric gram-negative rods and, on occasion, enterococci

Clinical Findings

  • Related to infected organ


  • Examination, laboratory tests, cultures, and CT scan


  • Therapy should be directed both against anaerobes and gram-negative aerobes

  • Antibiotics that are reliably active against B fragilis include

    • Metronidazole

    • Moxifloxacin

    • Tigecycline

    • Chloramphenicol

    • Carbapenems (ertapenem, imipenem, meropenem)

    • Ampicillin-sulbactam

    • Ticarcillin-clavulanic acid

    • Piperacillin-tazobactam

  • Resistance to cefoxitin, cefotetan, and clindamycin is increasingly encountered

  • Table 33–6 summarizes the antibiotic regimens for management of

    • Moderate to moderately severe infections (eg, patient hemodynamically stable, good surgical drainage possible or established, low APACHE score, no multiple-organ failure)

    • Severe infections (eg, major peritoneal soilage, large or multiple abscesses, patient hemodynamically unstable), particularly if drug-resistant organisms are suspected

Table 33–6.Treatment of anaerobic intra-abdominal infections.

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