Bacteroides fragilis resistance to clindamycin in vitro

Clindamycin resistance in Bacteroides fragilis was examined in 507 strains isolated from 1973 to 1981. Three groups were recognized: highly susceptible (minimum inhibitory concentration [MIC] less than or equal to 0.125 microgram/ml), intermediately susceptible (MIC = 0.25 to 4 micrograms/ml), and highly resistant to (MIC greater than or equal to 8 microgram/ml). The incidence of high-level resistance (1.8%) had not changed during this period. Only 8 of 17 isolates reputed to be highly clindamycin resistant that were referred to our laboratory proved to be highly resistant (MICs greater than or equal to 32 microgram/ml), whereas the other 9 were intermediately susceptible. Analysis of 2- and 10-microgram clindamycin disks for determining the susceptibility of B. fragilis revealed a high false-resistance rate with the 2-microgram disk, most errors occurring with the intermediate group. There was no false resistance with the 10-microgram disk. When disk diffusion susceptibility of B. fragilis is employed, we recommend the 10-microgram disk to predict accurately the susceptibility of B. fragilis to clindamycin.