A recent outbreak of streptococcal throat infections in Pittsburgh, Pennsylvania, involved bacteria that are resistant to erythromycin, a drug used to treat sore throats. Nearly half of the 'group A' Streptococcus (GAS) bacteria analyzed by researchers had developed resistance to the drug. GAS bacteria cause strep throat, rheumatic fever, and other infections.

Judith M. Martin, of Children's Hospital of Pittsburgh, and colleagues made the discovery during an ongoing study of streptococcal infections among children attending an elementary school in Pittsburgh. Using DNA fingerprinting techniques, the researchers determined that a single strain of GAS bacteria, called emm 6, was responsible for the recent outbreak.

The researchers analyzed 1,794 throat cultures obtained from 100 children between October 2000 and May 2001. Three hundred and eighteen cultures from 60 of the children were positive for GAS bacteria. Almost half of these (48 percent) were resistant to erythromycin.

The researchers further tested 100 isolates from the local community in the spring of 2001 and found that 38 percent were resistant to erythromycin. Such a high percentage of resistant isolates from one community has only been reported in GAS outbreaks in Europe and Japan, but not in the U.S., according to the researchers. The findings are published in The New England Journal of Medicine.

In previous studies carried out in the U.S., the rates of resistant isolates have been about 2 to 3 percent, says Martin. Based on GAS studies done in other parts of the world, she adds, US researchers should expect to find an increase in the frequency of drug-resistant GAS strains.

Among the many antibiotics used to treat strep throat, the drugs of choice are penicillin and amoxicillin. For patients who are allergic to these, doctors typically prescribe erythromycin, a type of macrolide antibiotic. Based on the new findings, the researchers are recommending that doctors not use this class of antibiotics to treat routine cases until more information is available.