• Streptococcus pyogenes;
  • macrolide antibiotics;
  • erythromycin resistance

Objective: To assess the incidence of resistance to erythromycin and to the three other macrolide antibiotics most extensively used in Italy (azithromycin, clarithromycin and roxithromycin) among clinical strains of Streptococcus pyogenes freshly isolated from throat cultures of pediatric patients in an area of Central Italy.

Method: Two sets of isolates were examined. The strains of the first set (n= 100) were collected according to a protocol admitting only throat swabs from untreated patients with symptoms of acute pharyngotonsillitis. The second set (n= 180) consisted of strains isolated from throat cultures during the routine activity of diagnostic laboratories, no particular protocol being applied.

Results: A trimodal distribution of strains was observed in relation to their macrolide susceptibility levels: two clusters were constituted by highly susceptible and highly resistant strains, respectively; a third, middle cluster consisted of strains displaying low-level resistance (or even intermediate susceptibility, in a minority of isolates, to clarithromycin). The distribution of individual isolates in the three modal clusters was the same with all four drugs. Both MIC ranges and MIC50s almost overlapped in the isolates of the two sets, whereas MIC90s were far higher in the strains of the second set (4 μg/mL for clarithromycin, 8 μg/mL for erythromycin and azythromycin, and 16 μg/mL for roxithromycin) than in those of the first (0.125 μg/mL for all four drugs). Resistant strains were 5% among the isolates of the first set and three times as many among those of the second.

Conclusions: The lower incidence of macrolide resistance recorded in the first set is probably more reliable: the threefold incidence observed in the second set may be overestimated due to the lower frequency of strains involved in drug-responsive infections and to the increased occurrence of strains from unsuccessfully treated patients.