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

Abstract

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.

A Rapid and High-Throughput Screening Approach for Methicillin-Resistant Staphylococcus aureus Based on the Combination of Two Different Real-Time PCR Assays

“…The surplus value of the GeneOhm MRSA assay was demonstrated because MRSA was cultured in 79.4% of all samples that tested positive by the GeneOhm MRSA assay, while the percentage was much lower when samples were cultured directly following the SMM panel (24%). Nevertheless, it should be stressed that positive results of both real-time PCR assays should always be confirmed using culture, since false-positive interpretations can be observed in the SMM panel and the GeneOhm MRSA assay because of a mixture of MRCNS and MSSA or a SCCmec-cassette with a mecA dropout, respectively (21)(22)(23).…”

Rapid Detection of Methicillin-Resistant Staphylococcus aureus (MRSA) and Methicillin-Susceptible Staphylococcus aureus (MSSA) Using the KeyPath MRSA/MSSA Blood Culture Test and the BacT/ALERT System in a Pediatric Population

“…This observation has been attributed to amplification of the SCCmec target in the absence of the mecA gene (the ''empty cassette'' phenomenon). 12 Data in the literature suggest that matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) may accurately detect S aureus directly from positive blood cultures. 13 However, MALDI-TOF MS is not FDA-approved nor capable of reliably performing antibiotic susceptibility testing at this time.…”

Ward-Specific Rates of Nasal Cocolonization with Methicillin-Susceptible and -Resistant Staphylococcus spp. and Potential Impact on Molecular Methicillin-Resistant Staphylococcus aureus Screening Tests

“…Hence, the amplification detects the presence of the SCCmec element only when it is inserted in the S. aureus genome, thus eliminating interference due to MRCoNS. Nevertheless, it had been demonstrated that this detection method can still give a falsepositive result in the presence of S. aureus isolates harboring an SCC element lacking the mecA gene; such isolates are designated mecA dropout isolates (12,13). These SCC-positive, methicillin-susceptible S. aureus (MSSA) isolates are misidentified as MRSA by commercial screening tests.…”