Macrolide resistance in group A streptococci

Two hundred and twenty one Streptococcus pyogenes isolates collected from t hroat swabs of untreated children with uncomplicated pharyngotonsillitis li ving in two centres situated in the north of Italy were tested to evaluate their macrolide resistance phenotype. Isolates were also typed for T protei n and assayed for opacity factor (OF) and protease production. Resistance t o macrolides was found to be similar in the two centres. Fifty-one point tw o per cent of Torino strains and 43.5% of Pinerolo strains were not inhibit ed by erythromycin. Resistant strains belonged to one of three phenotypes: CR, constitutive resistance (37.9 and 42.5% in Torino and Pinerolo, respect ively); IR, inducible resistance (40.9 and 17.5%); NR, new resistance pheno type (21.2 and 40%). All the resistant and some of the susceptible strains were analysed by pulsed-field gel electrophoresis and genomic patterns were defined on the basis of band size and number. Five DNA profiles were found among erythromycin-resistant strains: three patterns characterized the NR resistance phenotype and one each the IR and CR phenotypes. The distributio n of resistant strains according to their genomic patterns appears to be re lated to the resistance phenotype and only in some cases to the T serotype of bacteria. We conclude that the S. pyogenes strains analysed are genetica lly heterogeneous and therefore the high rate of erythromycin resistance ob served is not caused by the spread of a single clone nor is it related to a particular serotype.