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Declining macrolide resistance in Streptococcus pyogenes in Portugal (2007-13) was accompanied by continuous clonal changes.

OBJECTIVES
Macrolide resistance among Streptococcus pyogenes [group A streptococci (GAS)] in Portugal decreased between 1999 and 2006 and this decrease was accompanied by alterations in the prevalence of macrolide resistance phenotypes and clonal composition of the population. The aims of this study were to determine the macrolide resistance rate, resistance phenotypes and clones of GAS recovered from pharyngitis in 2007-13 in Portugal.


METHODS
Antimicrobial susceptibility was tested by disc diffusion. Macrolide-resistant isolates were characterized by emm typing, T typing, PFGE profiling and MLST, and the presence of macrolide resistance determinants was determined by PCR.


RESULTS
We found continuing changes in macrolide resistance phenotypes and a persistent decline in overall erythromycin resistance, from 10% in 2007 to 1% in 2013. During this period there was a marked increase in emm11-ST403 cMLSB isolates, the disappearance of the emm3-ST315 M lineage and changes in the prevalence of previously identified GAS clones. Unexpectedly, the decline in erythromycin resistance and the decreasing prevalence of the MLSB phenotype were accompanied by a high consumption of long-acting and intermediate-acting macrolides, known to select for resistance and particularly for the erm(B) gene.


CONCLUSIONS
The continuous decline in macrolide resistance detected since 2000, accompanied by a high clonal instability, emphasizes the importance of considering factors other than antibiotic consumption in explaining the prevalence of macrolide-resistant GAS.