Methicilin-resistant Staphylococcus aureus also known as MRSA may have become a sexually transmitted disease among gay men, researchers discovered.

Sexually active gay men in San Francisco are 13 times more likely to be infected than the general population, researchers have said in the Annals of Internal Medicine.

This is a new strain of MRSA, which is even harder to treat known as a variant of MRSA USA300. USA300 has been discovered early in 2001. The new strain is more resistant to three or even four classes of widely used antibiotics than the common MRSA.

“This particular clone is resistant to at least three other drugs, clindamycin, tetracycline and mupirocin,” Dr. Henry F. Chambers, who also led the study, said in a telephone interview.

MRSA causes deep and stubborn skin infections and has been named the most common cause of skin infections treated in the U.S.’ emergency rooms. The disease can rarely develop in lethal invasive infections such as pneumonia or sepsis (blood poisoning).

According to chemical analyses, bacteria are spreading among gay communities of San Francisco, researchers reported.

“Once this reaches the general population, it will be truly unstoppable. That’s why we’re trying to spread the message of prevention,” said Binh An Diep, a researcher at the University of California, San Francisco of California who led the study.

The study found that 1 in 588 residents living within the Castro neighborhood 94114 ZIP code area is infected with the variant. That compares with 1 in 3,800 people in San Francisco, according to statistical analyses based on ZIP codes.

“We probably had it here first, and now it is spreading elsewhere. This is a national problem and San Francisco is at the epicenter,” said Diep.

Surgical drainage and several classes of antibiotics, which are very expensive, can treat the disease.

What is strange is that staph infections had never been linked to sexual activity until last year. New York City physicians traced three instances of staph infection apparently spread by sexual contact early last year. Their report was published in February in the journal Clinical Infectious Diseases.

The study reports the bacteria seemed to be spread most easily through anal intercourse but also through casual skin-to-skin contact and touching contaminated surfaces. The San Francisco researchers suggested that scrubbing with soap and water might be the most effective way to prevent skin-to-skin transmission, particularly after sexual activities. In gay men, the strain causes abscesses and infection in the buttocks and genital area.

According the a report of the federal Centers for Disease Control and Prevention, various forms of MRSA are causing 95,000 of these more costly and potentially life-threatening infections. The disease killed about 19,000 Americans in 2005, most of them in hospitals, according to a report published in October in the Journal of the American Medical Association.

This new strain is rising along with the resurgence of syphilis, rectal gonorrhea and new HIV infections partly because of changes in beliefs about the severity of HIV and an increase in risky behaviors, such as illicit drug use and having sex that abrades the skin, Diep’s team wrote.

“Your likelihood of contracting each of these diseases increases with the number of sexual partners that you have. The same can probably be said for MRSA,” Diep said.

The study was supported by the CDC, the US Public Health Service, Pfizer and the University of California at Berkeley and at San Francisco.