Methicillin-resistant Staphylococcus aureus (MRSA) is a bacterium responsible for difficult-to-treat infections in humans. It may also be referred to as multidrug-resistant Staphylococcus aureus or oxacillin-resistant Staphylococcus aureus (ORSA). MRSA is by definition a strain of Staphylococcus aureus that is resistant to a large group of antibiotics called the beta-lactams, which include the penicillins and the cephalosporins.
MRSA is a resistant variation of the common bacterium Staphylococcus aureus. It has evolved an ability to survive treatment with beta-lactam antibiotics, including methicillin, dicloxacillin, nafcillin, and oxacillin. 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. Visitors to patients with MRSA infections or MRSA colonization are advised to follow hospital isolation protocol by using the provided gloves, gowns, and masks if indicated. Visitors who do not follow such protocols are capable of spreading the bacteria to cafeterias, bathrooms, and elevators.
The organism is often sub-categorized as community-acquired MRSA (CA-MRSA) or health care-associated MRSA (HA-MRSA) although this distinction is complex. Some have defined CA-MRSA by characteristics of patients who develop an MRSA infection while other authors have defined CA-MRSA by genetic characteristics of the bacteria themselves. The first reported cases of community-acquired MRSA began to appear in the mid-1990s from Australia, New Zealand, the United States, the United Kingdom, France, Finland, Canada, and Samoa, notable because they involved people who had not been exposed to a health-care setting. In 1997, four fatal cases were reported involving children from Minnesota and North Dakota. Over the next several years, it became clear that CA-MRSA infections were caused by strains of MRSA that differed from the older and better studied health care-associated strains. The new CA-MRSA strains have rapidly become the most common cause of cultured skin infections among individuals seeking emergency medical care in urban areas of the United States. These strains also commonly cause skin infections in men who have sex with men, athletes, prisoners and soldiers. However, in a 2002 report about CRSA, many cases were children who required hospitalization.