Methicillin-resistant Staphylococcus aureus – MRSA (also called multidrug-resistant Staphylococcus aureus) is a bacterium responsible for several difficult-to-treat infections in humans. MRSA is any strain of Staphylococcus aureus that has developed resistance to beta-lactam antibiotics, which include the penicillins (methicillin, dicloxacillin, nafcillin, oxacillin, etc.) and the cephalosporins. The development of such resistance does make MRSA infection more difficult to treat with standard types of antibiotics and thus more dangerous.
MRSA is especially troublesome in hospitals and nursing homes, where patients with open wounds, invasive devices, and weakened immune systems are at greater risk of infection than the general public.
Patient screening upon hospital admission, with nasal cultures, prevents the cohabitation of MRSA carriers with non-carriers, and exposure to infected surfaces.  In some countries hospitals screening for MRSA is performed in every patient  and all NHS surgical patients, except for minor surgeries previously checked for MRSA.