Drug-resistant germs are on the rise in the United States, and experts predict a sharp increase in the strains of a dangerous form of strep that can overcome two common antibiotics.

By the summer of 2004, as many as 40 percent of the strains of Streptococcus pneumoniae could be resistant to penicillin and erythromycin, researchers warn. That form of strep causes thousands of cases of meningitis, sinusitis, ear infections and pneumonia every year.

Researchers based at the Harvard School of Public Health studied reports from sites in eight states, measuring how common the drug resistance was in 1996 and how it increased by 1999.

Penicillin resistance rose from 21.7 percent of strep strains in 1996 to 26.6 percent in 1999, and for erythromycin it increased from 10.8 percent to 20.2 percent, the team reports in a paper appearing in today's online edition of the journal Nature Medicine.

The report provides further support for arguments against unnecessary antibiotic use, said Marc Lipsitch, one of the researchers.

For years, public health experts have warned that overuse of antibiotics -- often when they are not needed -- is leading to more drug-resistant bacteria.

For example, many people demand antibiotics when they have a cold, even though colds are caused by viruses, which are not affected by antibiotics.

Just last month the Food and Drug Administration announced plans to require a new warning on antibiotics, pointing out that overuse makes them less effective.

Doctors must be sure a patient has a bacterial infection, not a virus with similar symptoms, before prescribing antibiotics, the warnings say. The government estimates that half of the 100 million antibiotic prescriptions written in physician offices each year are unnecessary.

The new research "puts it on the doctor's plate," said Donald Low of Mount Sinai Hospital in Toronto. "Can we change the future? Yes, there are things we can do, but can we do them quickly enough?"

Doctors must use antibiotics the right way, in the proper dose, he said, and encourage vaccination, which reduces illness and thus the need for antibiotics.

Vaccines for that form of strep are recommended for infants and older people, the two groups most likely to get the infection.

Germs resistant to just one of the antibiotics can still be killed by the other, so the researchers project either a slow growth or a decline for these bacteria.

But using a new statistical formula that takes into account how the bacteria are transmitted, the researchers forecast rapid growth for strains of S. pneumoniae that are resistant to both of the antibiotics, reaching 40.6 percent by the summer of 2004. That's up from 8.6 percent in 1996.

"That's a bit of a chilling message," said Low, who was not part of the research team. "Within a year and a half, we'll know whether they're right or wrong." If the technique works, it will be a valuable tool in the future, he added.

Allison McGeer, also at Mount Sinai, added: "We have always known this is what will happen to resistance unchecked, and there is no reason to believe that their predictions will not come true."