OBJECTIVES

The increase in erythromycin resistance among Streptococcus pyogenes isolates is a cause for concern. We analysed trends in macrolide resistance, phenotypes, genotypes, resistance determinants and transposons among erythromycin-resistant S. pyogenes isolates collected from adults in a Barcelona hospital (1993-2008).

METHODS

Antibiotic susceptibility was studied by microdilution. Molecular typing was performed by PFGE, emm typing and multilocus sequence typing (MLST). Macrolide resistance genes and those related to the Tn916 family of transposons were detected by PCR.

RESULTS

Ninety-nine (18.3%) of 541 isolates were erythromycin resistant. Erythromycin resistance rates progressively increased from 0% (0/24) in 1993-1994 to 34.2% (50/146, P < 0.001) in 2001-2004, then falling to 7.4% (8/108, P = 0.02) in 2007-2008. Sixty-six erythromycin-resistant isolates were available for molecular studies. Of these, 26 had an M phenotype [mef(A)] and 40 had an MLS(B) phenotype [erm(B), n = 33; and erm(TR), n = 7]. Among M-phenotype isolates, the most frequent genotypes (88.5%) were emm4-ST39, emm6-ST382 and emm75-ST49, whereas genotypes emm11-ST403, emm28-ST52 and emm25-ST350 accounted for 72.5% of MLS(B)-phenotype isolates. Twenty-five isolates harboured both erm(B) and tet(M) genes related to the Tn916 family of transposons, Tn6002 being the most frequent. Ten isolates (10.1%) were ciprofloxacin non-susceptible, related to the emm6-ST382 clone with a ParC S79A change.

CONCLUSIONS

The peak of macrolide resistance rates among S. pyogenes observed in the 2001-2004 period was associated with an increase in the MLS(B) phenotype caused by the spread of emm11-ST403 and emm28-ST52 clones harbouring transposons of the Tn916 family. However, erythromycin resistance rates decreased significantly in the 2007-2008 period.