Two-year surveillance of emm-types and macrolide resistance of paediatric group A streptococcal pharyngitis isolates in the central part of Italy

Objectives. Surveillance of antimicrobial resistance and global dissemination of clones with specific emm-types is an important issue in GAS epidemiology, especially in relation to the design of a vaccine against the M protein. Methods. 584 GAS isolates were collected in central Italy during winter-spring 2012 and 2013, mainly from pharyngotonsillitis. We determined their emm-type according to CDC. Benzylpenicillin, erythromycin, telithromycin, clindamycin, tetracycline, linezolid, rifampicin, quinupristin/dalfopristin, and levofloxacin susceptibilities were tested by disc diffusion following the EUCAST guidelines. Results. 25 emm-types were recorded, nine of which accounted for almost 75% of the isolates (emm12, 89, 1, 4, 6, 3, 44, 5, 29). The overall distribution of emm-types between 2012 and 2013 was significantly different (C2-test P < 0.001) with major contributions given by emm-types 1, 3, and 89 (Fisher’s exact test P < 0.05). Resistance towards erythromycin, clindamycin, and tetracycline was observed in 8%, 4.8%, and 4.6% of cases, respectively. Macrolide resistance was mainly associated with emm-types 2, 4, 11, and 12. An important decrease in prevalence of macrolide resistance from 9.6% to 6% was recorded between 2012 and 2013. Conclusion. In view of the overall emm-distribution, the 26-valent vaccine would have covered 75% of the emm-types, while the 30-valent form would have approached 98% of coverage. Despite the slight decrease in macrolide consumption registered in the last ten years in Italy, the prevalence of macrolide resistance lowered consistently from 25-30% to less than 10%. The lower prevalence of some emmtypes or a decrease in the level of association of erythromycin resistance with some emm-types, may have well contributed to the overall drop in the observed prevalence of erythromycin resistance.