Abstract
High rates of erythromycin resistance among
Streptococcus pyogenes
strains have been reported in Italy in the last few years. In this study, 370 erythromycin-resistant (MIC, ≥ 1 µg/mL) Italian
isolates of this species obtained in 1997-1998 from throat swabs from symptomatic patients were typed by analyzing
Sma
I macrorestriction fragment patterns by pulsed-field gel electrophoresis (PFGE).
Among the typable isolates (
n
= 341; the genomic DNA of the remaining 29 isolates was not restricted by
Sma
I), 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-resistant group A streptococci circulating in Italy are polyclonal,
but the majority of them probably derives from the spread of a limited number of clones. In parallel experiments, the 370 test strains were characterized for the macrolide resistance phenotype: 80 were
assigned to phenotype cMLS, 89 to 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 data and the genotypic
results of PFGE analysis, the vast majority of the isolates assigned to individual PFGE classes belonging usually to a single phenotype of macrolide resistance. All of the 29 untypable isolates belonged
to the M phenotype. Further correlations were observed with tetracycline resistance.