The study, led by Dr. Judith Martin of Children's Hospital in Pittsburgh and coming out in Thursday's New England Journal of Medicine, is the latest evidence of a growing danger the medical community has been warning about for years, with little response from governments.
Scientists believe the practice of routinely feeding powerful antibiotics to livestock, along with the overuse of the drugs in humans, is allowing bacteria to develop a resistance to the medicines.
As a result, some infections that were once easily controlled by antibiotics are becoming increasingly difficult to contain.
Martin's team found 48 percent of the 318 throat cultures with group A streptococci taken in a single school from October 2000 to May 2001 were resistant to the drug erythromycin, which is usually given to children who are allergic to penicillin.
When the researchers randomly selected 100 strep cultures from outside the school between April and June of last year, 38 turned out to be resistant to erythromycin.
Such rates are unheard of in the United States, although they have been reported in Europe and Japan.
The finding "is a grave indication of the ability of bacteria to respond to the use of antibiotics," said Dr. Pentti Huovinen of the National Public Health Institute of Finland, in an editorial in the Journal.
In contrast, among 322 throat cultures with strep collected earlier -- between October 1998 and May 2000 -- none were resistant to erythromycin.
The Martin team said doctors should find out whether the problem has somehow become widespread in this country.
Huovinen said that because the Pittsburgh strain of strep may have already spread to other areas of the country, doctors need to be on the lookout for it elsewhere so they can prescribe the correct antibiotic against the bacterium, which causes tonsillitis, sore throats, skin infections and life-threatening septic infections.