Question
MRSA – mechanism of resistance, detection and treatment of cases and carriers, prevention.
Answer
MRSA (Methicillin-Resistant Staphylococcus aureus)
Mechanism of resistance: MRSA acquires resistance through the mecA gene (carried on a mobile genetic element, the Staphylococcal Cassette Chromosome mec, SCCmec), which encodes an altered penicillin-binding protein, PBP2a (PBP2’). PBP2a has a markedly reduced affinity for β-lactam antibiotics, allowing the organism to continue synthesizing its cell wall even in the presence of methicillin/other β-lactams (including all penicillins, cephalosporins, and carbapenems), conferring broad resistance to this entire antibiotic class.
Detection:
- Cefoxitin disc diffusion test — the preferred phenotypic screening method (cefoxitin is a better inducer of mecA expression than oxacillin/methicillin itself); a zone of inhibition below the breakpoint indicates resistance.
- Oxacillin agar screen test — growth on Mueller-Hinton agar containing oxacillin (with 4% NaCl) indicates resistance.
- PCR for the mecA gene — the gold-standard molecular confirmatory method, directly detecting the resistance determinant.
- Chromogenic agar media — for rapid screening, particularly in surveillance/carrier screening settings.
Treatment of cases: Vancomycin (traditionally first-line for serious MRSA infections) or Linezolid/Daptomycin for severe or vancomycin-resistant cases; Clindamycin or Cotrimoxazole may be used for less severe, susceptible skin/soft-tissue MRSA infections; Doxycycline is another oral option for uncomplicated skin/soft tissue infection.
Treatment/decolonization of carriers: nasal mupirocin ointment applied to the anterior nares (the most common carriage site) combined with chlorhexidine body wash, used for decolonization of identified carriers, particularly before high-risk surgical procedures or in outbreak settings.
Prevention:
- Hand hygiene — the single most important measure to prevent healthcare-associated transmission.
- Contact precautions — isolation/cohorting of known MRSA-colonized or infected patients, use of gloves and gowns by staff.
- Active surveillance screening of high-risk patients (e.g., ICU admissions, pre-surgical patients) to identify carriers.
- Environmental cleaning/disinfection of surfaces and shared equipment.
- Antimicrobial stewardship to reduce selective pressure driving resistance.