Which bacterium is commonly referred to as MRSA?

MRSA commonly refers to methicillin-resistant Staphylococcus aureus.

The name identifies resistant strains of S. aureus, a Gram-positive bacterium that often lives harmlessly on human skin or in the nose but can cause infections when it enters damaged tissue or the bloodstream. MRSA strains resist methicillin and related beta-lactam antibiotics, making some infections harder to treat than infections caused by methicillin-susceptible S. aureus, abbreviated MSSA.

MRSA was first reported in 1961, shortly after methicillin entered clinical use. It was initially associated mainly with hospitals, where surgery, open wounds, weakened immunity, and invasive devices increase infection risk. Community-associated and livestock-associated forms have also emerged, so MRSA is not exclusively a hospital organism.

The answer is sometimes made more precise as “methicillin-resistant Staphylococcus aureus,” because MRSA describes a resistant group of S. aureus strains rather than one separate species. It should not be confused with unrelated bacteria such as Streptococcus pyogenes or Pseudomonas aeruginosa.