Susceptibility trending of blood isolates of the Bacteroides fragilis group over a 12-year period to clindamycin, ampicillin-sulbactam, cefoxitin, imipenem, and metronidazole

Abstract

Numerous reports have described a steady overall increase in resistance among clinical isolates of the Bacteroides fragilis group to several antimicrobial agents, particularly clindamycin. Determination of resistance rates is significantly influenced by the number of isolates of each species within the B. fragilis group tested. Historically, the B. fragilis species has remained the most susceptible to most antimicrobials when compared to non-B. fragilis species. This study compares the effect of a gradually changing ratio of blood isolates of B. fragilis to non-B. fragilis species tested by broth micro-dilution over a 12-year period on selected antimicrobial agents. In 1987, the ratio of blood isolates of B. fragilis to non-B. fragilis was 68% to 32%; in 1991 it was 59% to 41%; and in 1999 it was 51% to 49%. Both metronidazole and imipenem showed the least changes because of their inherent high activity against all species. For clindamycin, decreases in susceptibility ranged from 84% to 64% for B. fragilis compared to 58% to 67% for non-B. fragilis species. Ampicillin-sulbactam showed a decrease in susceptibility in B. fragilis and non-B. fragilis species, but was highest in 1999 when the ratio of non-B. fragilis species was the highest. Overall resistance rates to cefoxitin varied from 8% to 25% during the testing years and was consistently higher among the non-B. fragilis species. These comparisons indicate that the ratio of B. fragilis group species isolated from the blood has changed over the last 12 years and has appreciably affected the resistance rates to some commonly used anti-anaerobic agents. Whether the noted changes in species isolation rates are a result of selective antibiotic pressure or other factors is yet to be determined.