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Streptococcus Resistance to Erythromycin Seems to Be Emerging

Kate Traynor

A study involving Pittsburgh-area schoolchildren revealed that an erythromycin-resistant Streptococcus pyogenes strain emerged and spread in the city last year, raising the possibility that macrolide-resistant strains of the bacterium exist elsewhere in the country and could complicate treatment.

Between October 1998 and May 2000, all 322 throat-swab cultures that had been obtained from the schoolchildren and had grown S. pyogenes, also known as group A streptococci, contained strains susceptible to erythromycin. But starting in January 2001, the specimens contained strains resistant to erythromycin.

By May 2001, a total of 153 erythromycin-resistant samples had been collected from 46 children. Genetic analysis showed that 44 of these children had been infected by the same clonal isolate of S. pyogenes, and the remaining two children harbored a closely related resistant strain.

In all, the resistant strain accounted for 48 percent of the S. pyogenes samples isolated during that school year. According to the researchers, whose findings appeared in yesterday's New England Journal of Medicine, a previously published study had estimated that about 2.6 percent of S. pyogenes isolates in the United States between 1994 and 1997 were resistant to erythromycin.

The discovery of the resistant microbe in the study group prompted the research team to examine whether similar antimicrobial resistance existed elsewhere in the community. Thirty-eight of 100 laboratory samples from children treated at the local pediatric hospital from April through June 2001 were isolates of S. pyogenes that were resistant to erythromycin.

Because infections with erythromycin-resistant S. pyogenes developed far more frequently than had been expected, the research team advised physicians to perform susceptibility testing before using a macrolide antibiotic to treat pharyngitis caused by S. pyogenes. Such a policy, the researchers noted, requires a shift from the current office-based practice of quickly testing for the presence of S. pyogenes antigens rather than culturing the bacterium and determining its susceptibility to various antiinfectives.

The Pittsburgh study took place at a private elementary school that enrolled about 285 children in kindergarten through eighth grade. In all, 2,200 throat-swab specimens were obtained during the first two years of the study and 1,794 during the third year. Fifteen percent of the cultures tested positive for S. pyogenes during the first two school years, and 18 percent of the samples collected during the third year contained the organism.

According to the report, the study physician prescribed an antimicrobial—penicillin V or amoxicillin—only when a child had symptoms of a respiratory-tract illness. Children who were allergic to penicillin received erythromycin initially, but when the erythromycin-resistant strain emerged, the physician changed to clindamycin.

None of the S. pyogenes samples obtained during the study contained clindamycin-resistant strains.

Diseases caused by infection with S. pyogenes include streptococcal pharyngitis (strep throat), scarlet fever, rheumatic fever, impetigo, and necrotizing fasciitis. Scientists at the University of Oklahoma last year published the complete genome sequence of S. pyogenes.