Question
MRSA.
Answer
MRSA (Methicillin-Resistant Staphylococcus aureus) refers to strains of S. aureus that have acquired resistance to methicillin and, by extension, essentially all beta-lactam antibiotics (penicillins, cephalosporins, and carbapenems) — a major cause of both hospital-acquired and community-acquired infections and an important target of infection-control policy.
Mechanism of resistance: MRSA carries the mecA gene (located on a mobile genetic element, the staphylococcal cassette chromosome mec, SCCmec), which encodes an altered penicillin-binding protein, PBP2a, with markedly reduced affinity for beta-lactam antibiotics — beta-lactams can no longer effectively bind and inhibit cell wall synthesis, conferring resistance across the entire beta-lactam class, not just methicillin specifically. A newer variant gene, mecC, has also been described in some strains.
Types:
- Hospital-acquired MRSA (HA-MRSA) — associated with healthcare settings, often multidrug-resistant (resistant to multiple additional antibiotic classes beyond beta-lactams), causing surgical site infections, catheter-related bloodstream infections, and ventilator-associated pneumonia.
- Community-acquired MRSA (CA-MRSA) — occurs in otherwise healthy individuals without healthcare contact, typically causes skin and soft-tissue infections, often carries the Panton-Valentine leukocidin (PVL) toxin gene (associated with severe necrotizing skin/soft-tissue infection and necrotizing pneumonia), and is often susceptible to more non-beta-lactam antibiotics than HA-MRSA strains.
Laboratory detection: cefoxitin disc diffusion testing (a more reliable surrogate marker for mecA-mediated resistance than testing oxacillin/methicillin directly); PCR detection of the mecA gene; automated susceptibility testing systems.
Treatment: vancomycin (mainstay for serious infections), linezolid, daptomycin, or clindamycin/co-trimoxazole for less severe skin/soft-tissue infections, depending on local susceptibility patterns.
Infection control: contact precautions, hand hygiene, and screening of high-risk patients are essential to prevent nosocomial spread.