In vitro activity of 6 antimicrobials against propionibacteria isolates from untreated acne papulopustulosa

In the present study, MIC values of 6 antibiotics were determined for 70 Pr opionibacterium acnes and 13 P. granulosum strains from 71 untreated acne p atients using the agar dilution method. The interpretation of in vitro resu lts is difficult because there are only poor data about the concentrations of antibiotics achievable in the sebaceous gland infundibulum. Based on bre akpoint concentrations according to DIN 58 944, no resistance was found aga inst chlortetracycline, doxycycline, minocycline, and chloramphenicol. In c ontrast, 11% of the P. acnes and 31% of the P. granulosum strains were not susceptible in vitro to erythromycin at a breakpoint concentration of 4 mu g/ml. The present study shows that these strains are not suppressed even at higher erythromycin concentrations up to 256 mu g/ml. Data from the litera ture reporting ineffective treatment of patients carrying erythromycin-resi stant P. acnes strains may be explained by our study. Concerning clindamyci n, resistance was found in only 6.8 % of P. acnes and in 0% of the P. granu losum strains. Since sensitivity was seen in 100% of the strains already at a concentration of 16 mu g/ml, a therapeutic effect may be possible.