Flagellin deficiency drives multi-drug resistance in Salmonella through biofilm adaptation and efflux pump activation.

Salmonella remains a leading foodborne pathogen of global public health concern. Of particular clinical relevance is the monophasic variant of S. Typhimurium, serotyped as S. 4,[5],12:i:-, which has emerged as an increasingly prevalent multi-drug resistance (MDR) strain worldwide. Characterized by the absence of phase 2 flagellar antigen expression, this variant has drawn significant attention due to its association with antimicrobial resistance. In this study, we systematically investigated the impact of flagellin deficiency on antibiotic tolerance in S. Typhimurium and S. Choleraesuis through the construction of isogenic mutants rSC0196 (S. Typhimurium UK-1(ΔfljBΔfliC)) and rSC0199 (S. Choleraesuis C78-3(ΔfljBΔfliC)). Our findings reveal that flagellin gene deletion confers enhanced antibiotic resistance in both serovars, despite significantly impairing their biofilm-forming capacity. Intriguingly, while biofilm biomass was reduced in the mutants, the residual biofilms displayed markedly increased antibiotic tolerance. Further studies demonstrated that flagellin deficiency significantly upregulated efflux pump activity in both mutant strains. These findings provide compelling evidence that flagellin deletion may serve as a key driver of MDR in S. 4,[5],12:i:- clinical isolates, potentially through dual mechanisms involving biofilm phenotypic alterations and efflux pump potentiation. This work not only advances our fundamental understanding of flagellin function in Salmonella pathogenesis but also provides valuable insights for the development of novel antimicrobial strategies targeting flagellin-mediated pathways.