Antimicrobial susceptibility patterns of the Bacteroides fragilis group in the United States, 1989

A national survey of Bacteroides fragilis group was continued in 1989 for the ninth consecutive year. Seven hundred thirty-nine isolates of B fragilis group from eight centers were tested for susceptibility to 14 antimicrobials. Sulbactam and clavulanic acid, beta-lactamase inhibitors, were tested at a constant concentration of 8 micrograms/ml and 2 micrograms/ml, respectively. Sulbactam was also tested in a fixed ratio of 1:2. Imipenem, ampicillin+sulbactam, and ticarcillin+clavulanic acid had resistance of less than 1% at breakpoints of 8 micrograms/ml, 16 micrograms/ml, and 64 micrograms/ml, respectively. At 32 micrograms/ml, resistance to cefoxitin, cefotetan, ceftizoxime, and ceftriaxone were 4%, 25%, 26%, and 46%, respectively. Clindamycin resistance was 10% at a breakpoint of 4 micrograms/ml. No isolates were resistant to chloramphenicol or metronidazole. Resistance for five B fragilis species to cefoxitin, ceftizoxime, and cefotetan varied greatly among both species and participating institutions. The addition of a beta-lactamase inhibitor increased the potency of the beta-lactam drugs tested as combinations. This finding suggests that beta-lactamase production is the major resistance factor in members of the B fragilis group.