POPULATION ANALYSIS OF SUSCEPTIBILITY TO METHICILLIN, VANCOMYCIN, AND THREE CEPHALOSPORINES IN TWO METHICILLIN‐RESISTANT STRAINS OF STAPHYLOCOCCUS EPIDERMIDIS

Abstract

Population analyses of susceptibility to methicillin, vancomycin, cephalothin, cefamandole and cefoxitin were carried out with 2 strains of S. epidermidis resistant to penicillin, streptomycin and methicillin. Two strains of S. aureus, 1 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 subpopulations highly resistant to methicillin, cephalothin, cefamandole and cefoxitin. The mutation rate was .apprx. 10-3-10-7. The 2 strains were homogeneous as regards vancomycin susceptibility. Methicillin was more penicillinase-stable than the cephalosporin. Treatment in vivo with cephalothin and vancomycin did not increase the frequency of highly resistant bacteria against any of the antibiotics; during treatment with cephalothin the majority population became 8 times more resistant to vancomycin and may explain why the treatment in vivo was not successful.