The Food and Drug Administration yesterday approved the first member of a new class of antibiotics to treat hospital patients infected with bacteria that are resistant to all other drugs.

The medicine, Synercid, is effective against a species of bacteria, Enterococcus faecium, that increasingly resists treatment with vancomycin, an antibiotic that hospital physicians have used for 30 years to fight infections when nothing else would work. The FDA also approved the new drug to treat complicated skin infections caused by the more familiar staph and strep bacteria.

The FDA's action comes as bacteria that used to succumb to familiar antibiotics such as penicillin are increasingly evolving new strains that cannot be killed by those drugs, making many infections harder to cure. For that reason, doctors have been urging pharmaceutical companies to develop antibiotics that work in new ways. Yesterday, they welcomed the news of Synercid's approval.

The drug is the first member of a family of antibiotics called streptogramins to be approved in the United States. It represents the first new class of antibiotics to appear on the U.S. market in 10 years, according to the FDA.

"I would say that it's always exciting to have another option available," said George M. Eliopoulos, an antibiotics researcher and member of the infectious disease division at Boston's Beth Israel Deaconess Medical Center.

At least three other new classes of antibiotics are being tested in patients, he added.

Synercid can only be given intravenously, and experts said yesterday that for most patients, running it into a small arm vein is so painful that it has to be delivered straight into one of the large veins in the chest or abdomen. It will cost an average of $250 to $300 per day and is expected to be used almost exclusively to treat certain seriously ill patients in hospitals and skilled nursing facilities.

The people whose lives may be saved by Synercid are likely to be desperately ill patients in intensive care units, including recipients of transplanted kidneys and livers, those who have had recent surgery and those whose immune systems have been weakened by cancer treatment, said Peter K. Linden, an associate professor of medicine and anesthesiology at the University of Pittsburgh School of Medicine.

Such patients are vulnerable to infection with E. faecium, a species of bacteria that lives in the human intestinal tract and can be spread by the hands of health care workers and even by contaminated stethoscopes and blood pressure cuffs. No one knows how many such infections occur annually, but doctors say there are thousands. Linden said about 50 percent of hospital-acquired strains of this bacteria are resistant to vancomycin.

In studies supported by Synercid's manufacturer, Rhone-Poulenc Rorer, the drug was given to 1,222 patients with vancomycin-resistant E. faecium infections who had no other treatment options. Although all patients were severely ill, the drug's overall effectiveness rate was 52 percent, said Sandra Kweder, acting director of the FDA's Office of Drug Evaluation 4.

The FDA also concluded that Synercid was safe and effective for treatment of skin and soft-tissue infections caused by Staphylococcus aureus and Streptococcus pyogenes, two common culprits in skin and wound infections. So far, however, these bacteria are almost always sensitive to vancomycin and usually to other antibiotics as well, so Synercid would not be the first-choice treatment, experts said.

For example, last month federal health officials reported the cases of four midwestern children who died of infections with Staphylococcus aureus resistant to the antibiotic methicillin. Those infections were acquired outside hospitals, however, and probably would have responded to vancomycin and a variety of other drugs if they had been caught early enough.

More than 2 million Americans acquire infections in hospitals each year, and about 1.4 million people in the United States receive vancomycin annually, said Bruce Lavin, associate director for U.S. medical affairs at Rhone-Poulenc Rorer. He said the company estimates that Synercid will capture 3 percent to 5 percent of the vancomycin market, and will be used primarily for patients whose infections are resistant to vancomycin, who are allergic to that drug, or who cannot tolerate it because of side effects.

Synercid is a combination of two drugs, quinupristin and dalfopristin, that bind to ribosomes--structures in bacteria that are part of the cell's protein-manufacturing machinery. The fact that Synercid has two components may make it harder for bacteria to evolve resistance to it, Eliopoulos said.

"We can say that there isn't a lot of resistance in the U.S. at the moment," he said. "I think that resistance will emerge and it's just our hope that it will emerge slowly and to a minimal degree."