Which gene is most commonly associated with MRSA resistance to methicillin?

Which gene is most commonly associated with MRSA resistance to methicillin?

• A. mecB
• B. mecA (and mecC)
• C. blaZ
• D. vanA

Answer: (B)

Methicillin resistance in Staphylococcus aureus comes from acquiring mecA or mecC, which encode an alternative penicillin-binding protein called PBP2a. PBP2a binds beta-lactams poorly, so even when the antibiotic is present, the bacteria can still synthesize their cell wall. This altered target is what lets MRSA survive methicillin and related drugs. The mecA/mecC genes are located on the mobile SCCmec element, which explains how resistance traits spread between strains. That’s why detecting mecA or mecC is the standard marker for MRSA. The beta-lactamase gene blaZ only adds penicillinase activity, which helps degrade some beta-lactams but doesn’t create the altered target that drives methicillin resistance. vanA is linked to vancomycin resistance, not methicillin. A less common mecB can contribute resistance in some contexts, but it is not the primary marker used to define MRSA in clinical practice.

Methicillin resistance in Staphylococcus aureus comes from acquiring mecA or mecC, which encode an alternative penicillin-binding protein called PBP2a. PBP2a binds beta-lactams poorly, so even when the antibiotic is present, the bacteria can still synthesize their cell wall. This altered target is what lets MRSA survive methicillin and related drugs. The mecA/mecC genes are located on the mobile SCCmec element, which explains how resistance traits spread between strains. That’s why detecting mecA or mecC is the standard marker for MRSA.

The beta-lactamase gene blaZ only adds penicillinase activity, which helps degrade some beta-lactams but doesn’t create the altered target that drives methicillin resistance. vanA is linked to vancomycin resistance, not methicillin. A less common mecB can contribute resistance in some contexts, but it is not the primary marker used to define MRSA in clinical practice.