Concern over drug-resistant staph

CHICAGO — When an HIV-infected patient walked into Dr. Daniel Berger’s office with a nasty sore on his wrist, the physician suspected the culprit was a bacterium known as MRSA.

The test results, however, were unexpected.

Yes, this was methicillin-resistant Staphylococcus aureus, but it was unresponsive to two medications that are recommended, mainstay treatments. Berger realized the already-formidable microbe had strengthened its defenses.

“I was quite concerned, needless to say,” said Berger, who since that incident two years ago has treated several other patients with similar infections.

With fascination and dismay, medical experts nationwide are watching the emergence of a new, even more extensively drug-resistant MRSA strain.

For now, the strain -- a variation of the MRSA circulating widely in community settings, outside hospitals -- has been documented primarily in urban gay communities in San Francisco and Boston, though anecdotal reports indicate it also has been seen in Chicago, New York, Los Angeles and Philadelphia.

And for now, the drug that doctors recommend most frequently for this type of infection, Bactrim, still is effective against it.

But scientists worry that the strain is poised to spread further. Some experts suggest it’s only a matter of time before the microbe adapts again and adds armor against this and other antibiotics.

“It’s like you’re going out on the battlefield and facing an enemy that keeps getting smarter and stronger, with only a few bullets left in your gun,” said Dr. John Quinn, a professor of medicine at Rush University Medical School.

Although most MRSA infections are mild or moderate, almost 100,000 become serious and result in 19,000 deaths each year, the Centers for Disease Control and Prevention reported last year.

The new multidrug-resistant strain is an altered version of USA 300, the most common form of community-acquired MRSA in the U.S. First identified in 2000, USA 300 isn’t the same bacterium that strikes patients in hospitals: It has a different genetic profile and is susceptible to more medications.

But with the acquisition of extra genetic material, this new strain of USA 300 became largely unresponsive to four drugs: clindamycin, ciproflaxcin, tetracycline and mupirocin.

Clindamycin and tetracycline are first-line treatments for community-acquired MRSA, according to Dr. Chip Chambers, a professor of medicine at San Francisco General Hospital.

Dr. Neil Fishman, chairman of the anti-microbial resistance work group for the Infectious Disease Society of America, said it was reasonable to predict this strain of USA 300 could become resistant to Bactrim in the next year.

If that happens, physicians can turn to other drugs, including the powerful antibiotic vancomycin. But those treatments are much more expensive and may need to be given intravenously, making treatment more costly and cumbersome.

The greatest worry scientists have is that USA 300 will continue to grow ever-more resistant to drugs.

The changes occur on a DNA molecule known as a plasmid, and the particular plasmid involved in this strain belongs to a family “shown to acquire resistance to vancomycin,” said Francoise Perdreau-Remington, director of the molecular epidemiology laboratory at UC San Francisco.

A crucial question is how quickly the new strain is likely to spread. USA 300 has proved adept at passing from person to person in gyms, locker rooms, schools and other settings. But often, acquiring drug resistance impedes a bacterium’s ability to jump from one host to another, Chambers noted.

There is evidence the strain has gained a notable foothold in gay communities in San Francisco and Boston, according to a recent report in the Annals of Internal Medicine. The study found that in 2004 and 2005, 1 in 3,800 San Francisco residents was struck by multidrug-resistant USA 300.

The paper speculates that sexual transmission may be responsible, at least in part, for infections in gay men.

But others question that hypothesis.

“There are a lot of alternative explanations for these findings,” Fishman said, including the possibility that gay men in these communities were more likely to visit doctors and get tests.

“Staph is transmitted [primarily] by skin-to-skin contact, and of course skin-to-skin contact can occur during sex,” said Dr. Rachel Gorwitz, a medical epidemiologist at the CDC. She said available evidence did not suggest MRSA was a sexually transmitted infection.