Ecological effects of oral antibiotics on the microflora of human skin.

The capacity of orally administered antibiotics to alter the resident microbial flora of the skin was evaluated after three weeks of treatment with tetracyclines, penicillins, erythromycin, and a lincomycin derivative. An increase in the percentage of resistant organisms was the most sensitive indicator that an antibiotic had reached the surface; conversion from a dominantly diphtheroid to coccal flora was another evidence of substantial surface levels. It was concluded that this mainly happened as a result of holocrine excretion via epidermal or sebaceous cells. Tetracyclines and erythromycin were instances of the latter effect. Penicillins did not materially alter the microflora, except for a slight increase in resistant cells. Significant reductions in the density of Cornebacterium acnes on the forehead were produced by three of four tetracyclines and the lincomycin derivative.