A Streptococcus suis Harmonized IdeSsuis Mac-1 Domain Genotype-Based Classification Proposal and Prevalence in European Isolates.

The classification systems currently in use for Streptococcus suis are primarily based on capsular polysaccharide (CPS) serotyping or multi-locus sequence typing (MLST). A genetic typing system based on the conserved Mac-1 domain of the IdeSsuis gene, which encodes an IgM-cleaving protease, could complement these approaches. Specifically, in this study, a genotyping system based on Mac-1 sequence homology was established, qualitative SYBR Green real-time PCR (qPCR) assays for standardized genotype classification were developed, and the global and European distribution of genotypes were analyzed. The Mac-1 classification system identified "two genotypes", one of which has "two subgenotypes", based on 427 unique sequences obtained from 4161 global sequences. GT1.1 was the most prevalent, having an amino acid similarity of 97.1%, followed by GT1.2 98.1%, and GT2 91.4%. The qPCR approach enabled reproducible, cross-laboratory-accessible typing. Among 5479 Streptococcus suis European isolates, 64% were GT1.1, 4.5% were GT1.2, and 29.6% were GT2, with a trend toward increasing GT1.1 detection rates between 2018 and 2023. Genotyping classification could complement current Streptococcus suis classification approaches to better support vaccine design and disease control strategies.