Genotypic and Phenotypic Characterization of Methicillin-Susceptible Staphylococcus aureus Isolates Misidentified as Methicillin-Resistant Staphylococcus aureus by the BD GeneOhm MRSA Assay

Twenty-three nasal swab samples that tested positive for methicillin-resistant Staphylococcus aureus (MRSA) on initial testing by the BD GeneOhm MRSA assay (BD-MRSA PCR; BD GeneOhm, San Diego, CA) were culture positive only for methicillin-susceptible S. aureus (MSSA) from an enrichment broth. The 23 recovered isolates were confirmed as MSSA by a variety of phenotypic methods, including the BD Phoenix automated microbiology system (BD Diagnostics, Sparks, MD), oxacillin screening agar (BD Diagnostics), BBL CHROMagar MRSA (BD Diagnostics), and a PBP2' assay (Denka Seiken Co., Tokyo, Japan); susceptibilities were determined by using Mueller-Hinton agar with oxacillin. All were positive by nuc PCR, specific for S. aureus, but negative for mecA with one exception. Isolates were characterized by using multiplex PCR methodology to determine structural types and variants (SCCmec typing); additional PCRs were performed for the detection of the ccr and mec complexes, the junkyard regions as well as the Panton-Valentine leukocidin. Pulsed-field gel electrophoresis was used to determine clonality. One phenotypic MSSA isolate contained an intact SCCmec. Twelve MSSA isolates tested positive for MRSA by the BD-MRSA PCR because of amplification of the mec priming site flanking the SCC insertion point, although these isolates lacked mecA. The 10 remaining isolates were not MRSA and tested as MSSA by phenotypic and genotypic assays. In our patient population, diagnostic and surveillance testing and subsequent infection control practices may be impacted by the frequency of these excision events when using the BD-MRSA PCR for MRSA detection.