Comparative Evaluation of Traditional Susceptibility Testing for MRSA with the PCR Approach

Abstract

Methicillin-resistant Staphylococcus aureus (MRSA) is a multi-drug resistant pathogen, which is responsible for increasing cases of serious diseases, including life-threatening diseases and nosocomial and community-acquired infections. Laboratory identification of MRSA is crucial and essential both for initiation of appropriate antimicrobial therapies and for effective infection control strategies that are designed to limit the spread of MRSA. In spite of the availability of commercial kits for MRSA detection in the market, the Clinical and Laboratory Standards Institute (CLSI) recommends the use of phenotypic methods, such as the disk diffusion method with oxacillin or with cefoxitin, as well as a serial dilution method with oxacillin. Nevertheless, some studies have shown that results obtained with such phenotypic methods are controversial. The aim of the study described in this paper was to comparatively evaluate the traditional susceptibility testing for MRSA with PCR as the gold standard assay. Analysis of collection (n = 68) isolates of Staphylococcus aureus revealed that the serial dilution method with oxacillin possessed the highest sensitivity (at 100%). In contrast, the disk diffusion methods with oxacillin and cefoxitin showed lower sensitivity (95.83%, 95% CI (78.81%-99.30%)). Furthermore, the borderline value of zone inhibition diameters for cefoxitin might be considered as a risk, and they may give false-susceptible result.