Changes in antimicrobial resistance phenotypes and genotypes in Streptococcus suis strains isolated from pigs in the Tokai area of Japan

Streptococcus suis strains isolated from porcine endocarditis and tonsils in the Tokai area of Japan during 2004–2007 and 2014–2016 (n=114) were tested for antimicrobial susceptibility and distribution of selected resistance genes. No strains showed resistance to penicillin, ampicillin, cefotaxime, meropenem, vancomycin, and levofloxacin. High resistance to tetracycline (80.7%), clindamycin (65.8%), erythromycin (56.1%), and clarithromycin (56.1%) was observed. In chloramphenicol and sulfamethoxazole-trimethoprim, there was a trend towards increased resistance between the first (2004–2007) and second (2014–2016) periods. tet(O) and erm(B) genes were the most frequently detected, and tet(M) and mef(A/E) genes were only detected in strains isolated during 2014–2016. These results indicate that chloramphenicol and sulfamethoxazole-trimethoprim resistance, and tet(M) and mef(A/E) genes emerged in S. suis of this area after 2014.