Genomic characterization and virulence of Streptococcus suis serotype 7 sequence type 373 of clonal complex 94

Streptococcus suis is a swine pathogen that also causes invasive infections in humans, leading to significant economic losses in pig production worldwide. Serotype 2 is the most pathogenic S. suis strain associated with human infections. However, non-serotype 2 strains isolated from humans have been reported globally. Here, we conducted a comparative genomic analysis of S. suis serotype 7-ST373 strains belonging to clonal complex 94, isolated from both humans and pigs, and assessed their virulence through mouse infection experiments. Genomic analysis revealed that S. suis serotype 7-ST373 strains harbor genomic islands 1-3 of the pathogenic S. suis clade. They also possess a high number of virulence-associated genes, similar to those of virulent serotype 2 strains, suggesting a high virulence potential. The antimicrobial resistance gene tet(O), which confers tetracycline resistance, was found in all ST373 strains, while erm(B), which confers macrolide resistance, was detected in most ST373 strains. However, the macrolide and lincosamide resistance genes lnu(B) and lsa(E) were found exclusively in a Thai human strain. Comparative genomics of ST373 strains with the virulent serotype 2 strain P1/7 identified 76 unique genes in ST373 strains, including 30 genes exclusively present in human ST373 strains. Mouse virulence experiments with two human ST373 strains (GX69 and STC2826) and one strain from a healthy pig (WUSS318) resulted in 100% mortality, classifying them as highly virulent. These findings indicate that serotype 7-ST373 strains demonstrate pathogenic potential and should be closely monitored.