Oral Antibiotic Treatment for Beta-Lactamase Positive Bacteroides fragilis Infections

Introduction to Recommended Treatments

Metronidazole is the definitive oral antibiotic of choice for beta-lactamase positive Bacteroides fragilis infections, with demonstrated bactericidal activity at easily achievable serum concentrations and near-universal susceptibility 1, 2

The Infectious Diseases Society of America recommends avoiding clindamycin for beta-lactamase positive B. fragilis due to resistance rates reaching 19% in B. fragilis isolates 1, 2, 3
The Infectious Diseases Society of America also recommends avoiding fluoroquinolones, including moxifloxacin, due to resistance rates of 27% documented in B. fragilis 1, 2, 3
Cefoxitin and cefotetan cannot be recommended due to increasingly resistant B. fragilis group organisms, with demonstrated worse outcomes when organisms are resistant 4, 5

Amoxicillin-clavulanate (Augmentin) is a beta-lactam/beta-lactamase inhibitor combination with activity against B. fragilis, and can be considered when metronidazole is contraindicated 4, 5
Moxifloxacin alone has moderate anaerobic activity, but should be reserved for patients with beta-lactam allergies, recognizing the 27% resistance rate 1, 2

Beta-lactamase production by B. fragilis renders all unprotected beta-lactams ineffective, necessitating either metronidazole or beta-lactamase inhibitor combinations 4, 5
Treatment failure with resistant organisms has been repeatedly demonstrated to worsen patient outcomes 4, 5