Detection of methicillin and mupirocin resistance in Staphylococcus aureus isolates using conventional and molecular methods:: a descriptive study from a burns unit with high prevalence of MRSA

Aims: To compare conventional phenotypic methods for the detection of methicillin and mupirocin resistance in Staphylococcus aureus in routine laboratory practice with reference to an established molecular method. Methods: This study was conducted on a selection of 65 isolates of methicillin resistant Staphylococcus aureus (MRSA) from a burns unit in India which is endemic for MRSA. The Kirby-Bauer and modified Stokes disc diffusion tests and the Vitek(TM) breakpoint minimum inhibitory concentration (MIC) were performed on all isolates using the presence of the mecA gene as the reference standard. Gel based and colorimetric polymerase chain reaction (PCR) assays were evaluated as molecular methods for the diagnosis of MRSA. A commercial latex agglutination test, the Mastalex(TM), was assessed for the detection of penicillin binding protein 2a (PBP2a), the mecA gene product. Conventional disc diffusion and molecular methods were investigated for the detection of mupirocin resistance. Results: Fifty one of 65 isolates were positive for the mecA gene. All three phenotypic methods showed high sensitivity (> 96.2%), whereas the specificity varied: 50% for Kirby-Bauer, 87.5% for modified Stokes, and 93.3% for Vitek. The colorimetric PCR was less cumbersome than the gel based PCR; there was complete concordance between both systems. The Mastalex(TM) kit showed good correlation with 1 PCR. One isolate was found to be mupirocin resistant and harboured the mupA gene. Conclusions: The specificity of routine laboratory tests for MRSA detection was variable. mecA gene detection, the "gold standard" to confirm ambiguous results, is difficult to perform in routine diagnostic laboratories. The Mastalex(TM) kit for the detection of PBP2a is an alternative that could be used in most laboratories. High level mupirocin resistance can be confirmed with genotypic methods.