What is MRSA Bacteria?

The name is a mouthful, but the meaning is simple. Staphylococcus aureus — "staph" for short — is a common bacterium that lives on the skin and in the nose of roughly one in three healthy people. Most of the time it causes no problem. When it does cause infection, it is usually a boil, a wound infection, or an abscess that clears with a routine antibiotic. MRSA is the version that no longer clears with those routine antibiotics — read more about the MRSA bacterium and how it enters the body.

MRSA is a bacterium, not a virus. Antivirals such as those used for flu or COVID-19 have no effect on it. Its resistance comes from a gene called mecA, which produces an altered target protein that penicillin-family drugs cannot bind to. Decades of antibiotic use in hospitals, in the community and in livestock farming have selected for these resistant strains.

Doctors usually split MRSA into two overlapping types. Healthcare-associated MRSA (HA-MRSA) spreads in hospitals, nursing homes and dialysis units, typically via catheters, surgical wounds and shared equipment. Community-associated MRSA (CA-MRSA) spreads outside healthcare — in gyms, prisons, households and sports teams — and more often shows up as painful skin abscesses in otherwise healthy people. Read more on what MRSA looks like, how it spreads and how it is treated. Parents can also read our guide to MRSA in children, babies and newborns.