Methicillin-resistant Staphylococcus aureus
Electron micrograph of MRSA
Scientific classification
Domain: Bacteria Kingdom: Bacteria Phylum: Firmicutes Class: Cocci Order: Bacillales Family: Staphylococcaceae Genus: Staphylococcus Species: S. aureus
Binomial name
Staphylococcus aureus Rosenbach 1884

Methicillin-resistant Staphylococcus aureus 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 . 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.

The organism is often sub-categorized as Community-Associated MRSA or Health Care-Associated MRSA although this distinction is complex. Some have defined CA-MRSA by criteria related to patients suffering from an MRSA infection while other authors have defined CA-MRSA by genetic characteristics of the bacteria themselves. CA-MRSA strains were first reported in the late 1990s; these cases were defined by a lack of exposure to the health care setting. In 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 healthcare-associated strains. ¹ The new CA-MRSA strains have rapidly spread in the United States to become the most common cause of cultured skin infections among individuals seeking medical care for these infections at emergency rooms in cities. These strains also commonly cause skin infections in athletes jail and prison detainees and soldiers.

MRSA is a resistant variation of the common bacterium Staphylococcus aureus. It has evolved an ability to survive treatment with beta-lactamase resistant beta-lactam antibiotics including methicillin dicloxacillin nafcillin and oxacillin. MRSA is especially troublesome in hospital-associated 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.