Antibiotic sensitivity of comedonal Propionibacterium acnes.

Previous studies on antibiotic MIC levels for P. acnes and P. granulosum have shown them generally sensitive to therapeutic levels in both blood and surface lipids. The clinical response of acne vulgaris to antibiotic therapy is slower than one would anticipate from in vitro studies. It is also delayed well beyond the time tetracycline is known to appear in surface lipids. The concentration of antibiotics in comedonal material could be important, but it is not known. To determine the sensitivity level required for such an assay, MIC studies on P. acnes and P. granulosum from comedonal material were carried out from 0.05 to 25 microgram/ml. The assay would need to detect 0.05 microgram/ml or less in comedonal material. Interestingly, two organisms were found to be resistant to one or more antibiotics at the 25 microgram/ml level and another had a MIC level of 12.5 microgram/ml for tetracycline.