Enhanced ability to colonize the skin: a possible explanation for the epidemic spread of certain strains of Staphylococcus epidermidis.

Experimental skin colonization was attempted on healthy volunteers using one epidemic and two non-epidemic strains of Staphylococcus epidermidis isolated from a bone marrow transplant unit. Although the three strains had similar biochemical reactions, they had different antibiograms and plasmid patterns, and the epidemic strain grew rather more slowly when in a mixture in broth. Two experiments involving sets of 5 volunteers were performed. The epidemic strain was mixed with one non-epidemic strain for experiment 1, and with the other for experiment 2. Each volunteer had an inoculum of a mixture of 10(7) cfu of each strain inoculated onto the antecubital fossae of both arms; one of the arms had had a prior treatment with chlorhexidine to see if this would prevent colonization. Quantitative skin cultures were continued until the test strains could no longer be isolated. Colonization occurred in all but one volunteer, and lasted from a few weeks to 17 months. Maximal counts of the epidemic strain were significantly higher than the maximal counts of the non-epidemic strains. Chlorhexidine had no effect in experiment 1, and caused a reduction in intensity and duration of colonization in experiment 2, although this did not achieve statistical significance. Plasmid patterns were unchanged throughout, but in two instances a variant of the epidemic strain that had lost resistance to methicillin and tobramycin was isolated together with the parent strain. The enhanced ability of the epidemic strain to colonize skin may be an important factor in allowing cross-infection.