Increased resistance of encapsulated Bacteroides fragilis to clindamycin

The antimicrobial susceptibility and in vitro growth curve of 4 nonencapsulated and 4 encapsulated isolates of Bacteroides fragilis were determined for clindamycin. The MIC of the nonencapsulated isolates was 1-2 dilutions less (0.062-0.25 microgram/ml) than the MIC for their encapsulated counterparts (0.25-0.5 microgram/ml). No difference was noted in the bacterial growth of the nonencapsulated or encapsulated isolates when incubated without clindamycin. The decline in the number of nonencapsulated isolates was significantly lower (p < 0.05) as compared to the encapsulated isolates when incubated with 0.1 or 0.4 microgram/ml of clindamycin. These results illustrate the higher susceptibility of nonencapsulated B. fragilis isolates to clindamycin as compared to their encapsulated counterparts. Since B. fragilis becomes more encapsulated during the infectious process, this finding underscores the advantage of early antimicrobial prophylaxis and therapy.