Salmonella Typhimurium StiP-mediated upregulation of membrane protein Alx drives complement evasion via CFI-dependent C3b degradation.

Invasive Salmonella Typhimurium (S. Typhimurium) causes lethal bloodstream infections (BSI), yet its molecular mechanisms remain unclear. We compared serum resistance and cellular invasive capabilities between invasive and non-invasive S. Typhimurium. Invasive strains exhibited significantly enhanced serum resistance (>17-fold survival in 75 % isolates) and HeLa cell invasive ability (minimum bacterial loads 3.93 × 105 CFU/mL) compared to non-invasive strains (<9.8-fold survival; maximum bacterial loads 1.47 × 105 CFU/mL). We next performed pan-genomic analysis and virulence gene database comparisons to elucidate the genetic mechanisms underlying robust serum resistance in invasive S. Typhimurium. We identified 15 differential genes unique to invasive S. Typhimurium, among which the StiP deletion strain (263ΔStiP) showed the greatest serum resistance reduction (2.49-fold). We further explored the role of StiP in host blood environment adaptation and found that 263ΔStiP displayed 3.12-fold reduced HeLa cell adhesion, 3.74-fold lower HeLa cell invasion, 1.92-fold decreased intra-macrophage survival, and 50 % reduced serum resistance versus wild-type 263 (WT263), collectively indicating that StiP is critical for host blood environment adaptation in invasive S. Typhimurium. Mechanistically, StiP upregulates the membrane protein Alx, which recruits complement factor I (CFI) to accelerate C3b degradation, thereby inhibiting classical complement pathway activation and enhancing invasive S. Typhimurium complement evasion. In vivo, 263ΔStiP-infected mice exhibited 5-, 7-, and 4-fold lower bacterial loads in blood, liver, and spleen (P < 0.001), respectively, with reduced pathological damage versus WT263. Thus, this study elucidates the StiP-Alx axis mediating complement evasion in invasive S. Typhimurium.