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 Propionibacterium acnes and 13 P.granulosum strains from 71 untreated acne patients using the agar dilution method. The interpretation of in vitro results is difficult because there are only poor data about the concentrations of antibiotics achievable in the sebaceous gland infundibulum. Based on breakpoint concentrations according to DIN 58,944, no resistance was found against chlortetracycline, doxycycline, minocycline, and chloramphenicol. In contrast, 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 micrograms/ml. The present study shows that these strains are not suppressed even at higher erythromycin concentrations up to 256 micrograms/ml. Data from the literature reporting ineffective treatment of patients carrying erythromycin-resistant P.acnes strains may be explained by our study. Concerning clindamycin, resistance was found in only 6.8% of P.acnes and in 0% of the P.granulosum strains. Since sensitivity was seen in 100% of the strains already at a concentration of 16 micrograms/ml, a therapeutic effect may be possible.