Synergy of Vancomycin plus Cefazolin or Cephalothin against Methicillin-Resistant Staphylococcus epidermidis

Abstract

The in vitro activity of cephalothin, cefazolin, and vancomycin against 25 isolates of methicillin-resistant Staphylococcus epidermidis was determined by means of a broth dilution technique with two sizes of inoculum. The size of the inoculum had a marked effect on the minimal inhibitory concentrations and the minimal bactericidal concentrations of all three antibiotics. With a small inoculum, 100% of the isolates were inhibited by 3.12 µg of vancomycin rrnl, 76% by 12.5 µg of cephalothin rml, and 64% by 12.5 µg of cefazolin rml. With a large inoculum 100% of the isolates were inhibited by 200 ~ of vancomycin rml, 40% by 12.5 µg of cephalothinjml, and 12% by 12.5 µg of cefazolin rml. As determined by a tube dilution checkerboard technique for both sizes of inoculum, the combination of vancomycin plus cephalothin was synergistic against methicillin-resistant S. epidermidis in 45 of 50 cases, and the combination of vancomycin plus cefazolin was synergistic in 39 of 50 cases. These data from in vitro studies suggest that these antibiotic combinations should be evaluated clinically in patients with severe infections caused by methicillin-resistant S. epidermidis.