EFFECTS OF TOPICAL ERYTHROMYCIN ON ECOLOGY OF AEROBIC CUTANEOUS BACTERIAL-FLORA

We have demonstrated previously that application of topical erythromyc in, an antibiotic commonly used for the treatment of acne, results in an increased density of cutaneous erythromycin-resistant (Em(r)) coagu lase-negative staphylococci; however, it is unknown if this increase r esults in an overall higher density of total cutaneous staphylococci o r if upon cessation of erythromycin use, Em(r) coagulase-negative stap hylococci remain at an increased density compared with the pretreatmen t density, To investigate this, 2% erythromycin or vehicle was applied to each subject's forehead (n = 225) twice a day by laboratory person nel for a period of 6 weeks, Samples were obtained for culture from th e forehead, anterior nares, and back of the subjects at baseline and a t weeks 6, 9, and 12 of the study, Cultures were performed on differen tial media. Plates into which erythromycin was incorporated (8 mu g/ml ) were used to identify Em(r) coagulase-negative staphylococci, The sp ecies of ail Em(r) coagulase-negative staphylococci were determined, a nd an antibiogram for 16 antibiotics was obtained, The baseline preval ence of Em(r) coagulase-negative staphylococci on the forehead and nos e was about 80% at the two study sites, whereas that on the back was 5 0%, The baseline density of Em(r) coagulase-negative staphylococci on the forehead, nose, and back was approximately 20% of the total flora, Following 6 weeks of erythromycin treatment, the prevalence of Em(r) coagulase-negative staphylococci on the forehead and nose was nearly 1 00% and the densities were 73 and 62%, respectively; the prevalence an d density for the back were 78 and 42%, respectively, The most prevale nt erythromycin resistance gene expressed by the Em(r) coagulase-negat ive staphylococci was ermC, There was no increase in the numbers of St aphylococcus aureus, gram-negative rods, or yeasts, nor was there incr eased resistance to any other antibiotic except clindamycin. The densi ty of total aerobic organisms also remained static, There were no chan ges in the prevalence or density of Em(r) coagulase-negative staphyloc occi in the vehicle group, A statistically significant decrease in the prevalence and density of Em(r) coagulase-negative staphylococci in t he erythromycin group was observed within 3 weeks posttreatment and by 6 weeks posttreatment, the prevalence and density returned to baselin e values, These data demonstrate that the increased prevalence and den sity of Em(r) coagulase-negative staphylococci as a result of topical 2% erythromycin use are transient on both population and individual le vels.