Comparison of the bactericidal activity of clindamycin and metronidazole against cefoxitin-susceptible and cefoxitin-resistant isolates of the Bacteroides fragilis group.

Time-kill kinetic methodology was used to evaluate the bactericidal activity of cefoxitin, cefotetan, clindamycin, and metronidizole against cefoxitin-susceptible and cefoxitin-resistant isolates of the Bacteroides fragilis group. Overall, metronidazole was the most bactericidal agent, with all isolates being killed with less than or equal to 4 micrograms/ml at 24 hr. Clindamycin was the next most bactericidal agent, with 20 of 26 isolates being killed with less than 16 micrograms/ml. Six isolates with clindamycin MICs greater than or equal to 64 micrograms/ml were not killed at 24 hr, with concentrations as high as 256 micrograms/ml. Cefoxitin and cefotetan were the least bactericidal agents tested. Seven isolates with MICs of greater than or equal to 64 micrograms/ml to each agent demonstrated a lack of killing at 24 hr, with concentrations of the respective agent as high as 256 micrograms/ml. At concentrations with either agent of 32 micrograms/ml, the remaining 19 isolates were killed at 24 hr. Of the six B. fragilis isolates resistant to clindamycin, four were also resistant to both cefoxitin and cefotetan. We conclude that in hospitals with cefoxitin-resistant B. fragilis group isolates, metronidazole would provide appropriate therapy.