In-vitro effects of penicillin and clindamycin on the expression of Streptococcus pneumoniae capsule

The effects of subinhibitory concentrations of penicillin or clindamycin were evaluated in 20 isolates of Streptococcus pneumoniae that were fully susceptible to penicillin and in 20 isolates that were of intermediate resistance. All isolates were capsulate and susceptible to clindamycin. After incubation in one-half of the MIC of clindamycin, 17.5% of isolates retained a capsule, compared to 87.5% after incubation with one-half of the MIC of penicillin. Clindamycin appears to be superior to penicillin in reducing the expression of the capsule by S. pneumoniae.