MRSA

MRSA, methicillin-resistant Staphylococcus aureus, is any strain of the common skin and nose bacterium S. aureus that has become resistant to methicillin and, with it, to almost every beta-lactam antibiotic, the family that includes the penicillins and most cephalosporins. Resistance usually comes from the mecA gene, carried on a mobile stretch of DNA, which codes for an altered penicillin-binding protein called PBP2a. The drugs bind this protein poorly, so the bacterium keeps building its peptidoglycan cell wall even while the antibiotic is present.

Methicillin came into use around 1960, and resistant strains were reported within about two years. MRSA first spread in hospitals and nursing homes, where it infects surgical wounds, catheter sites, the bloodstream and the lungs. Since the 1990s, community-associated strains have also spread among healthy people through skin-to-skin contact and shared items, for example in sports teams and crowded housing. Many people carry MRSA without illness. Most infections are boils and abscesses of the skin, but MRSA can also cause pneumonia, bone infection, sepsis and death.

Diagnosis needs a sample grown in a laboratory or tested for mecA. Treatment includes draining abscesses and antibiotics that still work, such as vancomycin or linezolid, chosen with susceptibility testing.