9.3 Confirming mechanisms in Gram-positive organisms
9.3.1 MRSA strains of S. aureus
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Can you recall why it is important to discriminate between different types of MRSA in S. aureus, i.e. mecA derived versus hyper-production of β-lactamase?
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Only genuine MRSA strains, i.e. those with an altered penicillin binding protein encoded by mecA or mecC genes, are of major public health and infection control importance. These are the strains likely to lead to treatment failure with β-lactam antibiotics.
Common screening methods for MRSA from swabs include colorimetric tests using chromogenic agar or mannitol salt agar. However, other tests such as the disk diffusion type tests or automated systems can be used, for example for clinical samples from invasive infections (PHE, 2020). Cefoxitin is now the recommended antimicrobial to use to confirm MRSA as it is less affected by conditions than oxacillin or methicillin and also detects mecC strains. Therefore, cefoxitin resistance confirms the MRSA phenotype.
9.2.3 Carbapenemases in practice
