How is MRSA detected phenotypically and/or genotypically in AST workflows?

• A. Genotypically by detection of mecA/mecC
• B. Phenotypically by oxacillin/cefoxitin resistance
• C. Both phenotypic and genotypic methods are used
• D. MRSA cannot be detected via AST

Answer: (C)

MRSA detection in AST workflows relies on identifying resistance to beta-lactam antibiotics caused by the mecA/mecC genes, using either phenotypic tests, genotypic tests, or both. Phenotypically, labs assess susceptibility to oxacillin or cefoxitin because cefoxitin is a strong inducer of the mecA-mediated resistance and provides a reliable readout in disk diffusion or MIC testing. A MRSA phenotype appears as growth in the presence of these agents or an elevated MIC, interpreted according to guidelines. Genotypically, tests look for the mecA or mecC genes (often by PCR) or for the PBP2a protein, which is the product of mecA. While mecA/mecC presence typically confirms MRSA, phenotypic tests can be influenced by expression levels, heteroresistance, or testing conditions, and some rare mecA/mecC variants may escape simple phenotypic detection. Conversely, genotypic testing might flag a potential MRSA when phenotypic expression is weak or absent under test conditions. Using both approaches provides robust, accurate detection, guiding effective therapy and helping with infection control decisions.