Non-penicillin -susceptible and linezolid-non-susceptible Streptococcus suis clonal complex 233 from humans

Streptococcus suis is an important swine pathogen and a zoonotic pathogen responsible for systemic diseases in humans. In 2021, an outbreak of non-penicillin-susceptible, optrA-harbouring S. suis serotype 2 sequence type (ST) 1656, belonging to clonal complex 233 (CC233), associated with the consumption of raw pork dishes, was reported in northeastern Thailand. This study aimed to characterize the genetic relationship between non-penicillin-susceptible, optrA-harbouring S. suis CC233 isolates from the outbreak and post-outbreak periods in 2021. We investigated the characteristics of 18 S. suis CC233 strains isolated post-outbreak in 2021 in Thailand using serotyping, multilocus sequence typing, and antimicrobial susceptibility testing. S. suis strains belonging to ST1656 were subjected to whole-genome sequencing. Comparative genome analysis was conducted between the ST1656 outbreak and post-outbreak strains. The analysis revealed that 17 out of 18 strains showed intermediate resistance to penicillin, with three strains exhibiting non-susceptibility to linezolid. PCR serotyping and MLST analysis showed that among the three linezolid-non-susceptible CC233 post-outbreak strains, two were serotype 2 ST1656 and one was serotype 14 ST233. ST1656 post-outbreak strains clustered with ST1656 outbreak strains but formed a separate branch, indicating that the ST1656 outbreak and post-outbreak strains shared the same ancestor. Differences in the genetic organization of the optrA gene, which is associated with linezolid resistance, were observed between outbreak and post-outbreak strains, suggesting that optrA may have been acquired from distinct origins. In contrast, the amino acid sequences of pbp1a, pbp2b, pbp2x, and mraY were identical between the ST1656 outbreak and post-outbreak strains, and their PBP and MraY amino acid sequences were homologous to sequences from non-penicillin-susceptible S. suis ST104 or ST233 strains. These findings highlight the zoonotic risk and emergence of antimicrobial-resistant S. suis CC233, underscoring the need for One Health surveillance and continued monitoring of penicillin and linezolid susceptibility in both swine and human populations.