Epidemiology, antimicrobial susceptibility, pathogenicity, and significance of Bacteroides fragilis group organisms isolated at Los Angeles County-University of Southern California Medical Center.

The epidemiology of species of the Bacteroides fragilis groups isolated at Los Angeles County-University of Southern California Medical Center was examined. In addition, frequency of resistance to six beta-lactam antibiotics (cefmetazole, cefotetan, ceftizoxime, imipenem, penicillin, and cefoxitin) and to clindamycin, chloramphenicol, and metronidazole was determined for each species. While B. fragilis was most commonly isolated, the other species of the B. fragilis group accounted for half of the isolates. Seven percent of 1,128 patients with infections due to species of the B. fragilis group were bacteremic. A review of bacteremic cases indicated that non-fragilis species were highly pathogenic. Resistance to clindamycin ranged from 8% to 22% among species and was most common among isolates of Bacteroides distasonis and Bacteroides thetaiotaomicron. Significant differences in antimicrobial activity were noted among the agents tested. Only imipenem, chloramphenicol, and metronidazole were predictably effective against non-fragilis species of the B. fragilis group. Prompt identification of species and susceptibility testing of clinical isolates of this group are needed if a newer beta-lactam agent or clindamycin is to be used for initial therapy.