Adaptation to Sublethal Sodium Hypochlorite Enhanced Biofilm Formation of <i>Salmonella</i> Enteritidis on Food-Contact Surfaces

Sodium hypochlorite (NaClO) is a widely used disinfectant in food processing to control foodborne pathogens. However, exposure to sublethal concentrations of NaClO may induce adaptive responses in bacteria, potentially affecting their persistence on food-contact surfaces. In this study, the effects of adaptation to sublethal NaClO on the biofilm-forming characteristics of Salmonella Enteritidis and the underlying regulatory mechanisms were investigated. NaClO-adapted cells exhibited significantly increased biofilm biomass on stainless steel 304 and glass compared with non-adapted cells. Compositional analysis of the biofilm matrix revealed that 48-h biofilms of NaClO-adapted cells contained higher levels of extracellular proteins, carbohydrates, and extracellular DNA (eDNA). Furthermore, adaptation to NaClO resulted in a significant reduction in motility, with swimming diameter decreasing from 7.73 ± 0.37 cm to 3.45 ± 0.42 cm. Additionally, NaClO-adapted cells exhibited enhanced cell surface hydrophobicity and a marked increase in auto-aggregation ability, with an auto-aggregation index of 39.12%, compared to 25.42% in non-adapted cells. Gene expression analysis revealed that adaptation to NaClO significantly upregulated key biofilm-related genes, including the biofilm master regulator csgD , cellulose synthase bcsA , global regulator csrA and quorum sensing luxS . In contrast, the flagellar biosynthesis gene fliA was significantly downregulated in NaClO-adapted cells, which was consistent with the observed reduction in motility. This study highlights the unintended risks related to improper NaClO application and provides critical insights for optimizing disinfection strategies to mitigate the persistence of biofilm-forming Salmonella Enteritidis and improve food safety.