A new variety of staph bacteria, highly resistant to antibiotics is spreading among gay men in San Francisco, Boston, New York and Los Angeles, researchers reported Monday.

The culprit is a form of MRSA, or methicillin-resistant Staphylococcus aureus, a bug that was once confined to hospitalized patients but, since the late 1990s, has been circulating outside medical settings, afflicting anyone -- not just gay community

The germ typically causes boils and other skin and soft-tissue infections and, despite its resistance to some drugs, is still treatable by surgical drainage and several classes of antibiotics. What is unusual in this case is the high percentage of infections - up to 40 percent - occurring in the buttocks and genitalia.

"Taking a shower after sexual contact may minimize contamination," said Dr. Chip Chambers, director of infectious diseases at San Francisco General, a co-author of the study. "Ordinary soap will do. It dilutes the concentration of bacteria. You don't need antibacterial soap."

Patients infected with HIV appear to run a higher risk of infection, but the study suggests that an unusually high percentage of gay men are being infected with the staph germ regardless of whether they are HIV-positive.

One factor that could be in play is a medical history of heavy use of antibiotics, which creates conditions for breeding drug-resistant strains. Any patient, HIV-positive or not, who has had high previous exposure to antibiotics might be more susceptible.

The new variant of USA300 is resistant to the antibiotics erythromycin, clindamycin, tetracycline, Cipro-like antibiotics and drugs in the penicillin family.

That still leaves a variety of antibiotics that will kill the new USA300 strain, but they tend to be more expensive and require intravenous drips. One common oral antibiotic, Bactrim, is still effective against it.