In a study published online by the journal Annals of Internal Medicine, the bacteria seemed to be spread most easily through anal intercourse but also through casual skin-to-skin contact and touching contaminated surfaces.

The authors warned that unless microbiology laboratories were able to identify the strain and doctors prescribed the proper antibiotic therapy, the infection could soon spread among other groups and become a wider threat.

The new strain seems to have "spread rapidly" in gay populations in San Francisco and Boston, the researchers wrote, and "has the potential for rapid, nationwide dissemination" among gay men.

The study was based on a review of medical records from outpatient clinics in San Francisco and Boston and nine medical centers in San Francisco.

The Castro district in San Francisco has the highest number of gay residents in the country, according to the University of California, San Francisco. One in 588 residents is infected with the new multidrug-resistant MRSA strain, the study found. That compares with 1 in 3,800 people in San Francisco, according to statistical analyses based on ZIP codes.

A separate part of the study found that gay men in San Francisco were about 13 times more likely to be infected than other people in the city.

The San Francisco researchers suggested that scrubbing with soap and water might be the most effective way to stop skin-to-skin transmission, particularly after sexual activities.

MRSA, for methicillin-resistant Staphylococcus aureus, was once spread chiefly in hospitals. But in recent years, a number of healthy people have acquired it outside hospitals.

Nearly 19,000 people died in the United States from MRSA infections in 2005, the Centers for Disease Control and Prevention has reported.

The infection can cause unusually severe problems, including abscesses and skin ulcers. The bacteria can invade through the skin to produce necrotizing fasciitis, giving them the popular name of flesh-eating bacteria. They can also cause pneumonia, damage the heart and produce widespread infection through the blood.

Among gay men in the study, MRSA was spread by skin contact, causing abscesses and infection in the buttocks and genital area.

The new strain is closely related to earlier ones. Both are known as MRSA USA300.

The strain is much more difficult to treat because it is resistant not just to methicillin, but also many more of the antibiotics used to treat the earlier strains, said Dr. Henry F. Chambers, an author of the new study.

The new strain contains a plasmid called pUSA03.

"This particular clone is resistant to at least three other drugs, clindamycin, tetracycline and mupirocin," Dr. Chambers said in a telephone interview.

Of the alternatives recommended by the C.D.C. and the Infectious Diseases Society of America, trimethoprim-sulfamethoxazole (Bactrim), clindamycin and a tetracycline, "this strain is resistant to two of those three," he added. "In addition, the new strain is resistant to mupirocin, which has been advocated for eradicating the strain from carriers."

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