Antimicrobial Susceptibility Testing And Rapid Diagnostics Practice Test

Methicillin resistance in Staphylococcus aureus is defined by the presence of the mecA gene, which encodes the low-affinity penicillin-binding protein PBP2a, allowing the bacterium to continue cell wall synthesis in the presence of beta-lactams. A mecC gene, a homolog of mecA, serves a similar function. Detecting mecA or mecC directly confirms the genotype responsible for MRSA resistance, making this the definitive test for methicillin resistance. By contrast, vanA encodes vancomycin resistance found in enterococci (and some staph strains but not driving methicillin resistance), blaZ encodes a beta-lactamase that can inactivate penicillin but does not cause methicillin resistance, and blaKPC encodes a carbapenemase in Gram-negative bacteria. So, mecA or mecC is the best indicator of MRSA methicillin resistance.