Resistance to macrolides in clinical isolates of Streptococcus pyogenes due to ribosomal mutations

Objective: Two clinical strains of Streptococcus pyogenes, 237 and 544, one isolated in Slovakia and the other in Croatia, that were resistant to azithromycin (MIC 8 and 2 mg/L, respectively) but susceptible to erythromycin (MIC 0.5 and 0.12 mg/L, respectively) did not contain any gene known to confer macrolide resistance by ribosomal modification (erm gene) or efflux [mef(A) and msr(A) genes]. The aim of the study was to determine the mechanisms of macrolide resistance in both strains. Methods: Portions of genes encoding ribosomal proteins L22 and L4, and 23S rRNA (domains II and V) in the two macrolide-resistant strains and in control strains susceptible to macrolides, were analysed by PCR and single-strand conformational polymorphism, to screen for mutations. The DNA sequences of amplicons from resistant strains that differed from those of susceptible strains, in terms of their electrophoretic migration profiles, were determined. Results: S. pyogenes 237 displayed a KG insertion after position 69 in ribosomal protein L4. S. pyogenes 544 contained a C2611U mutation in domain V of 23S rRNA. Conclusion: Mutations at a similar position in ribosomal protein L4 and 23S rRNA have been reported previously in macrolide-resistant pneumococci. This report shows that similar mutations can be found in macrolide-resistant S. pyogenes.