Changes in the susceptibility of Bacteroides fragilis group organisms to various antimicrobial agents 1979-1989.

The in vitro activity of metronidazole, chloramphenicol, clindamycin and 11 beta-lactam antibiotics against 135 clinical isolates of the Bacteroides fragilis group was compared. In addition, changes in the resistance patterns of members of the Bacteroides fragilis group isolated at the Hospital Universitario San Carlos in Madrid, Spain, between 1979 and 1989 were documented. The most active beta-lactam drugs were imipenem and beta-lactamase inhibitor combinations. In 1989, however, two strains were found to be resistant to imipenem and to all other beta-lactam agents tested. There was no emergence of resistance to metronidazole. Chloramphenicol was very effective: only one resistant strain was detected in 1979 and no chloramphenicol-resistant isolates were found during the rest of the study period. An outbreak of clindamycin resistance was noted in 1982, and the first cefoxitin resistant strains were recovered in 1985. The changing patterns of susceptibility of anaerobic bacteria to antimicrobial agents and the emergence of Bacteroides fragilis strains resistant to new beta-lactam agents suggest that periodic antimicrobial susceptibility tests should be performed in order to guide the selection of antimicrobial agents for therapy.