What is antibiotic-resistant S. aureus?

Methicillin-resistant Staphylococcus aureus, known as MRSA, is a type of Staphylococcus aureus that is resistant to the antibiotic methicillin and other drugs in the same class, including penicillin, amoxicillin, and oxacillin. MRSA is one example of a so-called "superbug," an informal term used to describe a strain of bacteria that has become resistant to the antibiotics usually used to treat it. MRSA first appeared in patients in hospitals and other health facilities, especially among the elderly, the very sick, and those with an open wound (such as a bedsore) or catheter in the body. In these settings, MRSA is referred to as health-care-associated MRSA (HA-MRSA).

MRSA has since been found to cause illness in the community outside of hospitals and other health facilities and is known as community-associated MRSA (CA-MRSA) in this setting. MRSA in the community is associated with recent antibiotic use, sharing contaminated items, having active skin diseases or injuries, poor hygiene, and living in crowded settings. The U.S. Centers for Disease Control and Prevention (CDC) estimates that about 12% of MRSA infections are now community-associated, but this percentage can vary by community and patient population.

MRSA infections are usually mild superficial infections of the skin that can be treated successfully with proper skin care and antibiotics. MRSA, however, can be difficult to treat and can progress to life-threatening blood or bone infections because there are fewer effective antibiotics available for treatment.

The transmission of MRSA is largely from people with active MRSA skin infections. MRSA is almost always spread by direct physical contact and not through the air. Spread may also occur through indirect contact by touching objects (such as towels, sheets, wound dressings, clothes, workout areas, sports equipment) contaminated by the infected skin of a person with MRSA. Just as S. aureus can be carried on the skin or in the nose without causing any disease, MRSA can be carried in this way also. In contrast to the relatively high (25%-30%) percentage of adults who are colonized by S. aureus in the nose (these people have staph bacteria present that do not cause illness), only about 2% of healthy people carry MRSA in the nose. There are no symptoms associated with carrying staph in general or MRSA in the nose.

A drug known as mupirocin (Bactroban) has been shown to be sometimes effective for treating and eliminating MRSA from the nose of healthy carriers, but decolonization (treating of carriers to remove the bacteria) is usually not recommended unless there has been an outbreak of MRSA or evidence that an individual or group of people may be the source of the outbreak.

More recently, strains of S. aureus have been identified that are resistant to the antibiotic vancomycin (Vancocin), which is normally effective in treating staph infections. These bacteria are referred to as vancomycin-intermediate-resistance S. aureus (VISA) and vancomycin-resistant S. aureus (VRSA).