TETRACYCLINE-RESISTANT PROPIONIBACTERIA FROM ACNE PATIENTS ARE CROSS-RESISTANT TO DOXYCYLINE, BUT SENSITIVE TO MINOCYCLINE

Antibiotic-resistant propionibacteria are being isolated with increasi ng frequency from antibiotic-treated acne patients. Minimum inhibitory concentrations (MICs) of three tetracyclines, extensively used in acn e therapy. were determined for 46 resistant and 19 sensitive propionib acterial isolates. Sensitive strains were inhibited by less-than-or-eq ual-to 1 mug/ml of all three tetracyclines. For every resistant strain tested, the MIC of tetracycline exceeded that of doxycycline which, i n turn, exceeded that of minocycline, The mean MIC for resistant strai ns was 20.61 +/- 4.56 mug/ml of tetracycline, 9.70 +/- 2.03 mug/ml of doxycycline and 1.95 +/- 0.3 5 mug/ml of minocycline. In order to dete rmine whether these strains could be inhibited by concentrations of mi nocycline achievable in vivo, serum levels of minocycline were determi ned in acne patients receiving either the recommended dose of 50 mg b. d. (20 males, 14 females), or twice this dose (21 males, 12 females). Serum levels were significantly higher (P < 0.001, Student's t-test) i n patients receiving 100 mg b.d. Males on 50 mg b.d. had significantly lower serum levels than females on the same dose (P < 0.05, Student's t-test). For all patients, the mean serum level on high-dose minocycl ine was 2-46 +/- 0.45 mug/ml, compared with 1.38 +/- 0.30 mug/ml on th e smaller dose. These results indicate that tetracycline-resistant pro pionibacteria should be considered clinically minocycline sensitive, i f patients who harbour such strains are prescribed 100 mg b.d. The rec ommended dose of minocycline for treating acne, especially in male pat ients, should be re-assessed.