CHANGING PATTERNS IN MACROLIDE RESISTANCE AND emm-TYPES IN Streptococcus pyogenes ISOLATED IN ITALY DURING 2012

Objectives. Surveillance of antimicrobial resistance and global dissemination of clones with specific emm-types is an important issue in GAS epidemiology also in relation to the design of a vaccine against the M protein. Methods. We have collected 218 GAS isolates, mainly from pharyngotonsillitis, in the Centre of Italy during winter-spring 2012 and determined their emm-type by sequencing. Antimicrobial susceptibility was tested by disc diffusion towards benzylpenicillin, erythromycin, telithromycin, clindamycin, tetracycline, linezolid, and rifampicin following the EUCAST guidelines. All resistant isolates were screened by PCR for the presence of the main corresponding genetic determinants of resistance. Results. The most frequent emm-types were emm-4, -1, -12, -89, -6, -44, -18, -29, -5, and -28 accounting for 80% of the isolates. Resistance towards erythromycin and telithromycin was observed in 10% of the isolates, clindamycin in 6%, and tetracycline in 4.6%. The 9 macrolide-ketolide resistant isolates were positive to mef(A), while the 12 macrolide-lincosamide-ketolide resistant isolates were positive to erm(B). In the latter group, one tetracycline susceptible and 9 resistant isolates were positive to tetM, which was present in another isolate resistant to tetracycline only. Macrolide resistance was mainly associated with emm-types 4, 11, and 12. Conclusion. The distribution and frequencies of the emm-types in contemporary Italian GAS have changed to the point that the vaccine coverage of the 26-valent formulation under development, which would have been about 77% ten years ago, would be 65% today (p<0.05). Against the slight decrease in macrolide consumption registered since the last ten years, the prevalence of macrolide resistance lowered consistently from 25-30% to 10%. This phenomenon may correlate to the disappearance or lower prevalence of some emm-types, such as emm2, emm77, and emm89.