Staphylococcus aureus is a common bacterium that lives on skin and in the noses of healthy people. If it gets into the body through a cut or scrape in the skin, it can cause generally mild, localized skin infections, including pimples, boils, and abscesses. But among surgical patients, or in cancer or AIDS patients, or elderly ones with chronic illnesses and weakened immune systems, staph can invade the blood and attack vital organs. These invasive staph infections are lethal in 20 to 25 percent of patients who develop them. A powerful antibiotic, methicillin, became the drug of choice for treating serious staph infections. But by the 1960s, the wily bugs had learned to resist methicillin, and a new strain of superbugs, called methicillin-resistant Staphylococcus aureus, or MRSA, became endemic in U.S. hospitals.

Until recently, however, doctors believed MRSA infections were limited primarily to people who had been confined in healthcare settings: hospitalized patients, people on dialysis or with indwelling catheters, or the elderly in nursing homes. But now, skin and soft tissue infections caused by MRSA have been showing up among high school athletes, children in day-care centers, prisoners, residents of rural Indian villages, and among members of the public never before believed to be vulnerable to MRSA.

What the researchers wanted to know: How common have these community-associated drug resistant staph infections become?

What they did: They reviewed data from 12,553 patients who had been diagnosed with an MRSA infection between 2001 and 2003 in Baltimore, Atlanta, and Minnesota, three areas that are part of a national MRSA survelliance project. Of these, 9,972 cases were immediately classified as having a healthcare-related MRSA infection. They attempted to interview more than 2,500 of the remaining patients and successfully did talk to more than 1,060 of them. About 280 of those actually had a healthcare-associated MRSA infection, leaving over 2,100 with a confirmed, or probable, community-associated MRSA infection.

What they found: MRSA infections have become far more common outside healthcare settings. In Atlanta, the rate of these MRSA infections was 26 cases per 100,000 people per year; in Baltimore, 18 per 100,000. (The Minnesota data were not comparable.) MRSA infections were significantly more likely in children under 2 years of age and more than twice as common in blacks, compared with whites in Atlanta. Racial differences were not significant in Baltimore. Though most of these were skin and soft tissue infections, 23 percent people who developed them had to be hospitalized.

What it means to you: Says the lead author, Scott Fridkin, M.D., an infectious disease expert at the federal Centers for Disease Control and Prevention: "For the average person, this study means that drug-resistant staph does occur in the community and they are at risk of getting it. If people have a skin infection, they need to go to their doctors and be tested for MRSA so they don't spread it to friends and family."

Caveats: The study is based on surveillance data, reported by hospitals to health departments. Cases of MRSA may be going untested, and unnoticed, so the rate of this bug in communities may be higher.

Find out more: The Centers for Disease Control and Prevention has a great deal of information on antibiotic resistance, including MRSA.

Read the article: Fridkin, S.K., et al. "Methicillin-Resistant Staphylococcus aureus Disease in Three Communities." The New England Journal of Medicine. April 7, 2005, Vol. 352, No. 14, pp. 1436–1444.

Abstract online: http://content.nejm.org