Methicillin-resistant Staphylococcus aureus (MRSA) is a bacterium responsible for difficult-to-treat infections in humans. It may also be referred to as multiple-resistant Staphylococcus aureus or oxacillin-resistant Staphylococcus aureus (ORSA). The organism is often sub-categorized as Community-Associated MRSA (CA-MRSA) or Hospital-Associated MRSA (HA-MRSA) depending upon the circumstances of acquiring disease, based on current data that these are distinct strains of the bacterial species.

MRSA is a resistant variation of the common bacterium Staphylococcus aureus. It has evolved an ability to survive treatment with beta-lactam antibiotics, including penicillin, methicillin, and cephalosporins. MRSA is especially troublesome in hospital-associated (nosocomial) infections. In hospitals, patients with open wounds, invasive devices, and weakened immune systems are at greater risk for infection than the general public. Hospital staff who do not follow proper sanitary procedures may transfer bacteria from patient to patient.