Non-serotype 2 Streptococcus suis isolates are a potential reservoir of AMR in Northern Thailand

Streptococcus suis is an important emerging zoonotic agent causing severe infection in humans, as well as causing global economic loss problems in the swine industry. Consumption of undercooked or improperly prepared pork meat or close proximity to pigs is generally believed to be the cause of most human zoonotic infections. An additional issue resides in the possible spread of antimicrobial resistance (AMR) genes across hosts during zoonotic infection. To date, this has not been examined extensively in S. suis isolates from South East Asia. In this study, 36 strains of S. suis were recovered from infected humans, from diseased pigs, and from healthy pigs in Chiang Mai province, Northern Thailand in 2015, and their genomes were sequenced. Despite >22% (8/36) of these isolates being from serotype 2, a clinically important lineage particularly associated with human pathogenesis worldwide, AMR carriage was more diverse and frequent in non-serovar 2 porcine isolates, with aadE, ant(6)-la, aph(3')-III, erm(A), erm(B), erm(T), mef(A), msr(D), lnu(B), lnu(C), tet(L), tet(O), optrA and catA8 mostly found in isolates from healthy rather than diseased animals. Overall, 75% (21/28) of all non-serotype 2 pig isolates carried >3 resistance genes, which was a much higher proportion than in serotype 2-strains (12.5%). Six plasmids, including pLFE1, pBM407, pAMbeta, Col440II, pLW043, and pCW7, were detected in 39.3% of non-serotype 2 isolates but absent in all serotype 2. β-lactam resistance genes were unexpectedly absent. We found the catA8 gene in a non-serotype 2 isolate from clinical healthy pig, which is, to our knowledge, the first report of the chloramphenicol resistance gene-catA8 in S. suis. In conclusion, many of the research focus is justifiably placed on S. suis isolates from serotype 2 due to their higher prevalence in human infection. However, our study indicates that non-serovar 2 S. suis isolates from farm animals should be considered as an important antimicrobial resistance reservoir for zoonotic S. suis and other clinically relevant streptococci.