The MIC values of DDAB for the different strains.

Salmonella enterica serovar 4,[5],12:i:-, a monophasic variant of Salmonella Typhimurium, has emerged as a common nontyphoidal Salmonella serotype to cause human foodborne disease, exhibiting antibiotic and multidrug resistance. In this study, we identified the isolates of S. 4,[5],12:i:- resistant to quaternary ammonium compounds (QACs) disinfectants, displaying elevated minimum inhibitory concentration (MIC) values (200 μg/mL) in Mueller-Hinton (MH) broth. The efflux pump SugE1 and SugE2 could be induced by didecyldimethylammonium bromide (DDAB) and found to be indispensable for S. 4,[5],12:i:- resistance to DDAB. The Hoechst 33342 dye accumulation and reduced ethidium bromide efflux in ΔsugE1, ΔsugE2 and ΔsugE1ΔsugE2 further confirmed the efflux function of SugE1 and SugE2. Moreover, DDAB inhibited the expression of Salmonella pathogenicity island 1 (SPI-1) to decrease the adhesion and invasion ability of S. 4,[5],12:i:- in IPEC-J2 cells, whereas the deletion of sugE2 increased the adhesion and invasion ability. In an in vivo mice model, the virulence of ΔsugE2 and ΔsugE1ΔsugE2 mutant strains were enhanced and showed significantly increased bacterial loads in the liver, spleen, and cecum. The ΔsugE2 and ΔsugE1ΔsugE2 mutant strains exhibited an enhanced ability to disrupt the intestinal barrier, leading to systemic infection. Finally, we discovered that intestinal extracts could induce sugE1 and sugE2 expression, subsequently suppressing SPI-1 expression through SugE2, mediating the Salmonella intestinal infection process. In conclusion, our findings provide the pivotal role of the SugE2 efflux pump in conferring resistance to DDAB disinfectants and influencing bacterial virulence in S. 4,[5],12:i:-.