Decrease in macrolide resistance and clonal instability among Streptococcus pyogenes in Portugal

Macrolide resistance among Streptococcus pyogenes (group A streptococci) in Portugal was stable during 1998-2003, but a rapid inversion in the dominant phenotypes was noted in the same period, with a sharp decrease in the proportion of isolates presenting the MLS(B) phenotype and a concomitant increase in isolates presenting the M phenotype. The characterization of group A streptococci recovered during 2004-2006, which is reported here, revealed that resistance was not stable during this period and that the decline in erythromycin resistance observed during 2004-2006 was due to a decrease in the prevalence of isolates presenting the M phenotype, while the proportion of isolates expressing the MLS(B) phenotype remained stable. Characterization by emm typing, T serotyping, pulsed-field gel electrophoresis (PFGE) profiling and multilocus sequence typing revealed a very diverse population. Several of the major PFGE clusters identified had already been found in the 1998-2003 study period, but others were found for the first time, e.g. T11/emm11/ST403, carrying the erm(B) gene, and T3/13/emm3/ST315, carrying the mef(A) gene. The clone defined as T12/emm12/ST36, previously found to be associated with mef(A), was now found to be predominantly associated with erm(B). The clonal dynamics of macrolide-resistant group A streptococci emphasizes the importance of considering factors other than antibiotic consumption in explaining the prevalence of resistant isolates.