Streptococcus suis serotype 9: a rising threat with potential meningitis-inducing capability, population structure, virulence genes, and antimicrobial resistance

Streptococcus suis ( S. suis ) is the principal etiological agent of swine streptococcosis and is an emerging zoonotic pathogen. In recent years, the prevalence of S. suis serotype 9 (SS9) has transitioned from primarily subclinical colonization to invasive disease, accompanied by a continuous increase in isolation rates across multiple countries. However, compared with that of serotype 2 (SS2), our understanding of SS9 remains markedly limited. In this study, we analyzed 7,913 S. suis isolates, among which SS9 was the second most prevalent serotype, accounting for 9.12% (722/7,913), followed by SS2 at 26.9% (2,132/7,913). We also isolated a highly virulent SS9 meningitis-associated strain and performed a comprehensive genomic analysis of 722 SS9 isolates. Population structure analysis revealed substantial genetic diversity within SS9, comprising 168 sequence types (STs) and 23 CCs, with ST16 representing the predominant lineage. The predominance of isolates from the brain and cerebrospinal fluid (37.89%), together with the lack of source-specific phylogenetic clustering, suggests a potential association of SS9 with central nervous system-related infection, particularly meningitis. Bioinformatics analysis revealed significant differences in the distribution of 17 virulence genes between brain-associated and nonbrain-associated isolates, indicating a potential association between these genes and meningitis. The most common antimicrobial resistance genes were tet(O) (54.29%) and erm(B) (62.05%), with 36.98% of the strains carrying both resistance to tetracycline and resistance to MLS B antibiotics. This study was designed to investigate whether the meningitis-associated pathogenicity of SS9 is related to the enrichment of specific virulence determinants and multidrug resistance-associated genomic features, thereby enhancing our understanding of the pathogenicity and resistance of SS9 and providing important data for improving disease control strategies and public health preparedness.