Decline in macrolide resistance rates among Streptococcus pyogenes causing pharyngitis in children isolated in Italy
OBJECTIVES To survey antibiotic resistance among Streptococcus pyogenes isolates collected from 2005 to 2012, to characterize those showing erythromycin resistance and to analyse the association of certain emm types with erythromycin resistance or susceptibility. METHODS Resistance determinants or mutations conferring erythromycin, clindamycin, tetracycline and fluoroquinolone resistance were analysed. All erythromycin-resistant isolates and a sample of erythromycin-susceptible isolates were emm typed. Multilocus sequence typing was performed for representative emm types. RESULTS Antimicrobial susceptibility was studied for 12 346 S. pyogenes isolates. Erythromycin, clindamycin and tetracycline resistance showed a decreasing trend. In 2012, 2.8% of isolates were erythromycin resistant versus 7.5% in 2005 and 11.7% in 2006. Although 21 clones were involved, 4 clones accounted for almost 90% of erythromycin-resistant isolates. The emm12/ST36 clone, carrying the mef(A) gene, was the predominant (41.1%) erythromycin-resistant clone, with an incidence peak in 2008, followed by a gradual decline. The M phenotype predominated each year except for 2005, when two of the main erythromycin-resistant clones (emm11/ST403 and emm28/ST52) harboured an erm(B) gene. Erythromycin resistance was significantly higher in adults than in children. Skin isolates showed the highest erythromycin resistance rate; among these, perianal isolates frequently belonged to the emm28/ST52 clone. The emm type was not a predictor of erythromycin resistance; however, most emm11 and emm12 were erythromycin-resistant isolates. Macrolide consumption was similar throughout the study period. Only two isolates with a high level of levofloxacin resistance were detected. CONCLUSIONS Resistance was mainly related to the circulation of emm12/ST36, emm11/ST403, emm28/ST52 and emm4/ST39 clones, all of which declined throughout the study period.