Emergence of antibiotic resistant strains of Bacteroides fragilis.

From 1981 to 1986, antibiotic susceptibility was tested by the agar dilution method in 534 isolates of Bacteroides fragilis organisms recovered from clinical specimens. Drugs evaluated included penicillin, ampicillin, tetracycline, clindamycin, metronidazole and chloramphenicol. Based on in vitro testing, tetracycline, ampicillin and penicillin were ineffective against B. fragilis because of high rates of resistance to these agents (74 per cent tetracycline resistant, 80 per cent ampicillin resistant and 89 per cent penicillin resistant). Throughout the five years of the study, 5.1 per cent of the organisms studied were resistant to clindamycin compared with 2.1 per cent resistant to metronidazole (p less than 0.01) and 0.9 per cent resistant to chloramphenicol (p less than 0.001). More important than the relative resistance rates to the various antibiotics is the observation that this is a dynamic process that is influenced by local practices of administering antibiotics. In 1981, the first year of the survey, no clindamycin resistant B. fragilis isolates were identified; by 1986, 7.8 per cent of the strains proved resistant to this antibiotic. Similarly, no metronidazole or chloramphenicol resistant organisms were demonstrated in 1981 or 1982; thereafter, resistance to these two drugs was noted with increasing frequency each year. The emergence of B. fragilis strains that are resistant to the antibiotics in current use mandates the surveillance of the local susceptibility data to identify important trends. When increasing rates of resistance are noted to one antibiotic, alternative regimens can be used.