Evaluation of phenotypic methods for methicillin resistance characterization in coagulase-negative staphylococci (CNS)

Coagulase-negative staphylococci (CNS) are the major cause of nosocomial infections. Methicillin-resistant strains are particularly important because they narrow therapeutic options. Detecting methicillin resistance among CNS has been a challenge for years. The objective of this study was to determine the accuracy of an agar screening test (0(.)6 and 4 jug oxacillin ml(-1)), disc diffusion and the automated MicroScan system to characterize methicillin resistance among CNS. One hundred and seventy five strains were analysed: 41(.)1% Staphylococcus epidermidis and 59(.)9% other species; 69(.)1% were mecA-positive. The results showed that the methods have optimal correlation with the detection of mecA gene for S. epidermidis, Staphylococcus hominis and Staphylococcus haemolyticus. However, accuracy of the tests is impaired when less common species are analysed. The only 100% accurate test was agar screening with 4 mug oxacillin ml-1.