TraC promotes complement evasion of Salmonella enterica serovar Choleraesuis by recruiting host C4b-binding protein.

Salmonella enterica serovar Choleraesuis (S. Choleraesuis) is a highly invasive zoonotic pathogen responsible for severe systemic infections, in which the complement system constitutes a major innate defense against circulating bacteria. However, the molecular basis underlying resistance of S. Choleraesuis to complement-mediated killing remains poorly understood. Here, whole-genome sequencing and pan-genome comparative analyses of S. Choleraesuis isolates exhibiting distinct serum-resistance phenotypes identified five candidate genes associated with enhanced serum survival. Among them, traC emerged as the most critical determinant. In S. Choleraesuis, deletion of traC reduced serum resistance by 73%, impaired adhesion and invasion to HeLa cell by 55% and 71%, respectively, and decreased intracellular persistence in RAW264.7 cells by 24-55%. In a murine infection model, loss of traC markedly attenuated systemic dissemination, resulting in significant reduction of bacterial loads in blood (80%), liver (32%) and spleen (63%). Genetic complementation of traC restored these phenotypes to near wild-type levels. Mechanistically, TraC promoted recruitment of the host complement regulator C4b-binding protein (C4BP), which may facilitate factor I-mediated C3b degradation, resulting in reduced C3b deposition on the bacterial surface and enhanced bacterial survival under serum exposure. Co-immunoprecipitation and GST pull-down assays further supported an interaction between TraC and C4BP, likely involving the N-terminal region of TraC. Furthermore, ectopic expression of traC in low-invasive S. Choleraesuis was sufficient to enhance serum resistance and suppress complement deposition. Collectively, these findings suggest that TraC contributes to complement evasion in invasive S. Choleraesuis and may facilitate bacterial adaptation during bloodstream infection by modulating host complement regulation.