Detection of methicillin resistance in coagulase-negative staphylococci by cefoxitin disc diffusion and oxacillin Etest. A study of consecutive bacteraemia isolates

Abstract

Coagulase-negative staphylococci (CoNS) are a significant cause of nosocomial bacteraemia and their susceptibility to beta-lactamase-stabile penicillins is unpredictable. To ensure appropriate antibiotic therapy reliable methods for detection of methicillin resistance (MR) are needed. The objectives of this study were to determine the frequency of MR in a set of CoNS from cases of monomicrobial bacteraemia and to evaluate two phenotypic assays for detection of MR, the 10 microg cefoxitin disk test on Iso-Sensitest agar using a semiconfluent inoculum and the oxacillin Etest. MR was determined by a commercial genomic mecA assay. Of 110 CoNS, 75 were mecA positive and 35 mecA negative. Using interpretive zone diameters R < 22 mm and S > or = 27 mm, the cefoxitin disk test had a sensitivity and specificity of 100%. A correct prediction was obtained for 86 isolates, while 23 were indeterminate (> or = 22 mm; <27 mm). Using CLSI's guidelines, sensitivity and specificity of the oxacillin Etest were 100% and 80%, respectively. A correct prediction was obtained for 102 isolates, while 7 mecA negative isolates were classified as resistant. Thus, the cefoxitin disk test and the oxacillin Etest performed with high accuracy and both seem to be suitable for routine use.