Quorum-Sensing Stimulation and Phytochemical Quenching Reshape Biofilm-Associated Gene Expression in Salmonella enterica

Quorum sensing (QS) influences biofilm formation, persistence and stress adaptation in Salmonella enterica. Although Salmonella does not synthesise acyl-homoserine lactones (AHLs), it can detect exogenous AHLs through the LuxR homolog SdiA, allowing it to respond to interspecies signalling cues in polymicrobial environments. This study investigated whether external QS stimulation and quorum-modulatory compounds reshape biofilm-associated transcriptional programmes in S. enterica serovar Enteritidis (SE) and S. Typhimurium ST14028. Biofilm formation was assessed using the crystal violet assay, while expression of QS-, biofilm-, adhesion-, motility- and invasion-associated genes (sdiA, csgD, flgG, fimA, rck, invA, bapA and hilA) was quantified using multiplex RT-qPCR and analysed by the {Delta}{Delta}Ct method, with 16S rRNA used for normalisation. In parallel, molecular docking was used to explore the predicted interaction of C8-HSL, established quorum-quenching agents and selected phytochemicals with the Salmonella SdiA ligand-binding region. Exposure to exogenous C8-HSL increased biofilm biomass and induced coordinated upregulation of QS- and biofilm-associated genes in both serovars, supporting the role of external AHL sensing in Salmonella biofilm regulation. In contrast, farnesol and furanone produced broad transcriptional repression accompanied by reduced biofilm biomass. Selected natural products, including epigallocatechin gallate (EGCG), thymoquinone, garlic extract, turmeric extract and aloe-emodin, produced moderate antibiofilm effects and partial downregulation of QS-associated transcriptional responses, suggesting possible interference with biofilm-regulatory signalling pathways. Molecular docking further supported this interpretation by identifying potential interactions between selected quorum-modulatory compounds and the predicted SdiA binding region, providing a plausible mechanistic basis for their observed biological effects. Notably, responses differed between SE and ST14028, indicating strain-dependent sensitivity to QS stimulation and quorum-modulatory treatments. Collectively, these findings suggest that exogenous AHL sensing contributes to strain-dependent transcriptional reprogramming of Salmonella biofilm-associated genes and that selected phytochemicals may act as preliminary quorum-modulatory candidates. This study supports further investigation of SdiA-mediated signalling as an anti-virulence target for reducing Salmonella persistence in food-associated and clinical environments.