Current status of antimicrobial susceptibility of clinical isolates of Streptococcus pyogenes in Japan: report of a countrywide surveillance study

The growing number of macrolide-resistant strains of Streptococcus pyogenes is an increasing problem worldwide. In this study, we evaluated 62 clinical isolates of S. pyogenes obtained from the upper respiratory tract. Susceptibilities to penicillins, cephalosporins, fluoroquinolones, macrolides, and carbepenems were determined by minimal inhibitory concentrations (MICs). Expressions of macrolide-resistance genes (mefA, ermB, and ermTR) were also examined, by polymerase chain reaction (PCR). All strains were susceptible to β-lactams. On the other hand, of the 62 S. pyogenes isolates, 6.5%, 6.5%, 6.5%, and 3.2% of the strains were resistant to azithromycin (AZM), roxithromycin (RXM), clarithromycin (CAM), and telithromycin (TEL), respectively. Four (6.5%) strains had a type of macrolide-resistance gene; there were two strains with ermB and two strains with ermTR, and these four strains were resistant to AZM, CAM (one strain was intermediately resistant), and RXM. Strains having ermB were resistant to TEL (MIC, ≥ 8μg/ml), while strains having ermTR were susceptible to TEL. Physicians and researchers need to take into consideration the macrolide resistance of some strains of S. pyogenes.