Incidence of erythromycin resistance in Streptococcus pyogenes: a 10-year study.

We studied the evolution of susceptibility of Streptococcus pyogenes isolated in our hospital from 1987 to 1996. Susceptibility to penicillin, ampicillin, cefotaxime, cefuroxime, imipenem, erythromycin, clindamycin, tetracycline, vancomycin, ciprofloxacin, rifampin, and chloramphenicol was determined by the National Committee for Clinical Laboratory Standards broth microdilution method. Differentiation of phenotypes of erythromycin-resistant strains was performed using the double-disc method. All isolates remained very susceptible in vitro to penicillin and all of the other beta-lactam agents tested. Between 1987 and 1995 the incidence of erythromycin resistant strains remained below 5%; the difference in the resistance rate between 1995 (2.6%) and 1996 (17.1%) was statistically significant. The macrolide resistance M phenotype was the most frequent. The isolation rates of tetracycline-resistant strains increased from 2.2% in 1987 to 11.2% in 1988. The marked increase in the incidence of erythromycin resistance observed in our area warrants periodic surveillance of antibiotic susceptibility of S. pyogenes isolates and emphasizes the need to control outpatient antibiotics. The preponderance of the M phenotype may have implications in the choice of antibiotic.