INHIBITION OF ERYTHROMYCIN-RESISTANT PROPIONIBACTERIA ON THE SKIN OF ACNE PATIENTS BY TOPICAL ERYTHROMYCIN WITH AND WITHOUT ZINC

Propionibacteria resistant to high concentrations of erythromycin [min imal inhibitory concentration (MIC) greater than or equal to 0.5 mg/ml ] are now commonly isolated from the skin of antibiotic-treated acne p atients. This double-blind study was carried out to assess the ability of 4% w/v erythromycin with and without 1.2% w/v zinc acetate to redu ce the numbers of erythromycin-resistant propionibacteria in vivo, and also to monitor the acquisition of resistant strains de novo during t herapy. Under laboratory conditions, erythromycin-resistant propioniba cteria were shown to be as sensitive to zinc acetate as fully sensitiv e strains. In vivo, the erythromycin/zinc complex and erythromycin alo ne produced highly significant reductions in total propionibacteria (P <0.001) and in the number of erythromycin-resistant strains (P<0.001 a t 8 weeks). After 12 weeks, resistant propionibacteria were reacquired , or acquired de novo, by three patients treated with erythromycin alo ne and four patients treated with the erythromycin/zinc complex. In co ntrast, changes in numbers of Micrococcaceae were slight and, after 12 weeks, erythromycin-resistant strains were predominant in both treatm ent groups. In vitro MIC determinations suggested that this finding mi ght be explained by the exceptionally high degree of erythromycin resi stance displayed by some staphylococcal strains (MIC > 4 mg/ml) and by the relative insensitivity of all staphylococcal strains to zinc acet ate. Erythromycin with and without zinc was clinically effective, and both preparations produced significant reductions in acne grade, and i nflamed and non-inflamed lesion counts (P<0.001). In particular, 11 of 12 patients who harboured > 10(3) c.f.u. erythromycin-resistant propi onibacteria/cm(2) skin pretreatment (seven on the erythromycin/zinc co mplex and five on erythromycin alone) showed clinical improvement, wit h a >50% reduction in acne grade and/or lesion count. These results sh ow that topical 4% w/v erythromycin with and without zinc eradicates e rythromycin-resistant propionibacteria in vivo, and is thus therapeuti cally effective in patients who harbour such strains.