SmaI macrorestriction analysis of Italian isolates of erythromycin-resistant Streptococcus pyogenes and correlations with macrolide-resistance phenotypes

High rates of erythromycin resistance among Streptococcus pyogenes strains have been reported in Italy in the last few years. In this study, 370 eryth romycin-resistant (MIC, greater than or equal to 1 mug/mL) Italian isolates of this species obtained in 1997-1998 from throat swabs from symptomatic p atients were typed by analyzing SmaI macrorestriction fragment patterns by pulsed-field gel electrophoresis (PFGE), Among the typable isolates (n = 34 1; the genomic DNA of the remaining 29 isolates was not restricted by SmaI) , 48 distinct PFGE types were recognized, of which 31 were recorded in only one isolate (one-strain types). Fifty-two percent of typable isolates fell into three type clusters and 75% into six, suggesting that erythromycin-re sistant group A streptococci circulating in Italy are polyclonal, but the m ajority of them probably derives from the spread of a limited number of clo nes. In parallel experiments, the 370 test strains were characterized for t he macrolide resistance phenotype: 80 were assigned to phenotype cMLS, 89 t o phenotype iMLS-A, 33 to phenotype iMLS-B, 11 to phenotype iMLS-C, and 157 to phenotype M. There was a close correlation between these phenotypic dat a and the genotypic results of PFGE analysis, the vast majority of the isol ates assigned to individual PFGE classes belonging usually to a single phen otype of macrolide resistance. All of the 29 untypable isolates belonged to the M phenotype, Further correlations were observed with tetracycline resi stance.