Phenotypic and Genotypic Characterization of Streptococcus pyogenes Isolates Displaying the MLS B Phenotype of Macrolide Resistance in Spain, 1999 to 2005

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Abstract
The aim of this study was to describe the genetic characteristics of Streptococcus pyogenes showing the MLS B phenotype of macrolide resistance from 1999 to 2005 in Spain and to highlight the substantial increase in these isolates in the last few years. The antimicrobial susceptibilities of 17,232 group A streptococci isolated from Madrid and Gipuzkoa from 1999 to 2005 were studied. The presence of the resistance genes ermA , ermB , mef , tetM , and tetO and the presence of the intTn and xis genes of the Tn 916 -Tn 1545 transposon family were studied in a sample of 739 MLS B -resistant isolates. The epidemiological relationships among these isolates were analyzed by emm typing, T typing, and multilocus sequence typing. Erythromycin resistance was found in 21.3% of the isolates analyzed (annual variation of 14.3% to 28.9%). Until 2003, most erythromycin-resistant isolates showed the M phenotype, but in 2004 and 2005, about 50% of isolates showed the MLS B phenotype. Among the MLS B -resistant isolates studied, 16 clones were identified. The most prevalent clone was a strange emm 11/T11/ST403 clone with a null yqiL allele. All but one of the 463 emm 11/T11/ST403 isolates carried the ermB , tetM , intTn , and xis genes. The second most prevalent MLS B -resistant clone was emm 28/T28/ST52, which comprised two subclones: one bacitracin-resistant, tetracycline-susceptible subclone carrying the ermB gene ( n = 115) and another bacitracin-susceptible, tetracycline-resistant subclone carrying the ermB and tetM genes ( n = 33). The rapid diffusion of these two clones, and especially of emm 11/T11/ST403, caused the large increase in MLS B -resistant S. pyogenes isolates in Spain, suggesting a potential ability for international dissemination.