Phenotypes and genotypes of macrolide-resistant Streptococcus pyogenes isolated in Seoul, Korea

The mechanisms of resistance to macrolides in 51 erythromycin-resistant clinical isolates of Streptococcus pyogenes collected from 1997 through 2003 in Seoul, Korea were evaluated. They were characterized by their antimicrobial susceptibility, phenotype (using triple-disc and induction tests), resistance genotype, emm genotyping (M typing) and phylogenetic analysis. Erythromycin resistance was observed in 23% of isolates. Inducible phenotype was the most common (iMLS, 51%, 26 strains), followed by the constitutive phenotype (cMLS, 31%, 16 strains) and the M phenotype (18%, 9 strains). Eight of twenty-six iMLS isolates exhibited the iMLS-C phenotype. The remaining 18 isolates gave small inhibition zones (< 12 mm) around all three discs, and mild blunting of the spiramycin and clindamycin zones of inhibition proximal to the erythromycin disc. They showed remarkable inducibility in erythromycin and clindamycin resistance. The MIC₉₀ of erythromycin and clindamycin rose from 8 to > 128 pg ml^-1 and from 0.5 to > 128 pg ml^-1, respectively. Their resistance characteristics did not fit into any known iMLS subtype reported so far in the literature. So, it was named as an iMLS-D, new subtype. All of these iMLS-D strains harboured the erm(B) gene, demonstrated the emm12 genotype, except one, and formed a tight cluster in a phylogenetic tree, with 89.2 to 100% sequence homology, suggesting that they are closely related. Nine of sixteen cMLS strains had the emm28 genotype, which had been reported to be associated with multiple drug resistance.