Tyrosine kinase Cps2C regulates the synthesis of capsular polysaccharides and plays a role in the virulence of <i>Streptococcus suis</i> serotype 2

<i>Streptococcus suis</i> is a gram-positive zoonotic pathogen that causes several serious diseases in humans and various animals. Among its virulence factors, capsular polysaccharides (CPS) play a particularly pivotal role in its pathogenicity. However, the regulatory mechanisms governing CPS synthesis remain elusive. In this study, we identified a gene located at the 5' end of the <i>cps</i> locus, <i>cps2C</i>, that encoded a tyrosine kinase. Deletion of <i>cps2C</i> resulted in significant changes in bacterial morphology that were characterized by a reduced colony size, an abbreviated chain length, and decreased CPS levels. Through tandem mass-labeled quantitative phosphoproteomic analysis, we discovered significant downregulation in the phosphorylation levels of numerous proteins in the Δ<i>cps2C</i> mutant, including key CPS synthesis-associated glycosyltransferases (Cps2F, Cps2H, Cps2J, and NeuB), cell division-related proteins (FtsZ, FtsE, SepF, and DivIVA), and components of the phosphotransferase system (EI, HPr, EIIABMan, and EIICGlu). Both Cps2F and Cps2H were confirmed to be the key substrates of Cps2C via in vitro kinase assays. In addition, deletion of <i>cps2C</i> in <i>S. suis</i> serotype 2 enhanced bacterial adherence to HEp-2 cells, reduced anti-phagocytosis properties, and attenuated pathogenicity in mice. Taken together, our data suggest that Cps2C regulates the capsular synthesis process of <i>S. suis</i> serotype 2, thereby affecting bacterial virulence. These results suggest that Cps2C may be a promising target for controlling <i>S. suis</i> serotype 2 infection.