Population analysis of susceptibility to methicillin, vancomycin, and three cephalosporines in two methicillin-resistant strains of Staphylococcus epidermidis.

Population analyses of susceptibility to methicillin, vancomycin, cephalothin, cefamandole, and cefoxitin were carried out with two strains of Staphylococcus epidermidis (S. epidermidis) resistant to penicillin, streptomycin, and methicillin. Two strains of Staphylococcus aureus (S. aureus), one sensitive to penicillin the other resistant, were used as controls. All strains were clinical isolates. From the strains of S. epidermidis it was possible to select sub-populations highly resistant to methicillin, cephalothin, cefamandole, and cefoxitin. The mutation-rate varied between c. 10(-3) and 10(-7). The two strains were homogeneous as regards vancomycin susceptibility. Methicillin was shown to be more penicillinase-stable than the cephalosporines. Treatment in vivo, with cephalothin and vancomycin did not increase the frequency of highly resistant bacteria against any of the antibiotics, but during treatment with cephalothin the majority-population became eight times more resistant to vancomycin and may explain why the treatment in vivo was not successful.