MRSA colonization commonly resides where and is resistant to which antibiotics?

• A. Nose; resistant to antistaphylococcal penicillins (methicillin, nafcillin, oxacillin)
• B. Lungs; resistant to macrolides
• C. Skin; resistant to cephalosporins
• D. Blood; resistant to all antibiotics

Answer: (A)

MRSA typically lives in the front of the nose (anterior nares), acting as a reservoir that can spread or lead to infection elsewhere. The defining resistance is to beta-lactam antibiotics, including methicillin and other antistaphylococcal penicillins such as nafcillin and oxacillin. This resistance comes from the mecA gene, which changes the target penicillin-binding protein (PBP2a) so these drugs bind poorly and fail to inhibit cell wall synthesis. Because of this, MRSA strains are not susceptible to these penicillins. The other sites listed aren’t the usual colonization sites, and while MRSA can share broader resistance patterns, the hallmark is resistance to these beta-lactam antibiotics due to mecA-mediated PBP2a.