SSU_RS09155, a virulence-associated gene encoding a putative membrane of unknown function, that is essential for...

Streptococcus suis is an important swine pathogen and zoonotic agent, with the capsular polysaccharide recognized as its major virulence factor. Recent genomic studies have identified several virulence-associated genes (VAGs), including SSU_RS09155, SSU_RS09525, and SSU_RS03100, as potential markers of pathogenicity. The current study evaluated the direct contribution of these genes to S. suis virulence using isogenic deletion mutants of the serotype 2 reference strain P1/7. In a murine infection model, ΔSSU_RS09525 and ΔSSU_RS03100 mutants displayed virulence comparable to the parental strain, whereas the ΔSSU_RS09155 mutant was significantly attenuated, showing reduced mortality and bacteremia. In vitro, the ΔSSU_RS09155 mutant demonstrated increased susceptibility to killing in whole blood and enhanced phagocytosis by murine macrophages, while no significant defects in capsular polysaccharide (CPS) expression or growth in rich medium and mouse and porcine plasma were observed. Complementation restored the resistance to the whole-blood killing phenotype, confirming the specific role of SSU_RS09155. Adhesion to and invasion of porcine tracheal epithelial cells were unaffected. In pigs, the natural host, the ΔSSU_RS09155 mutant was almost completely avirulent, with minimal to undetectable bacteremia. In silico analysis suggested that SSU_RS09155 encodes a hydrophobic, membrane-associated protein potentially involved in resistance to innate immune mechanisms, although the real function remains unknown. Together, these findings identify SSU_RS09155 as a critical virulence factor in S. suis serotype 2, whereas SSU_RS09525 and SSU_RS03100 appear dispensable under the experimental conditions tested.