Common food preservatives impose distinct selective pressures on Salmonella Typhimurium planktonic and biofilm populations

Food preservatives are crucial in controlling microbial growth in processed foods to maintain food safety. Bacterial biofilms pose a threat in the food chain by facilitating persistence on a range of surfaces and food products. Cells in a biofilm are often highly tolerant of antimicrobials and can evolve in response to antimicrobial exposure. Little is known about the efficacy of preservatives against biofilms and their potential impact on the evolution of antimicrobial resistance. In this study we investigated how the common food pathogen Salmonella enterica serovar Typhimurium responded to subinhibitory concentrations of four common food preservatives (sodium chloride, potassium chloride, sodium nitrite or sodium lactate) when grown planktonically and in biofilms. We found that each preservative exerted a unique selective pressure on S. Typhimurium populations grown planktonically and in a biofilm. Biofilm formation itself seemed to confer protection when exposed to each of the four preservatives, more so than previous exposure to sub-inhibitory concentrations of preservatives. There was a trade-off between biofilm formation and growth in the presence of three of the four preservatives, where prolonged preservative exposure resulted in reduced biofilm biomass and matrix production over time. Despite the differences in biofilm formation and preservative tolerance seen following three preservative stresses, they selected for mutations in global stress response regulators rpoS and crp. There was no evidence for any selection of cross-resistance to antibiotics after preservative exposure, and some evidence that antagonism between preservatives can be exploited in compound cocktails to reduce contamination in the food chain. HighlightsO_LIPreservative-specific evolutionary adaptation of Salmonella was shown over time. C_LIO_LIA trade-off between adaptation and biofilm formation was observed. C_LIO_LINo cross-resistance to antibiotics was seen after preservative exposure. C_LIO_LIMutations were found to be preservative-specific, with some common ones like rpoS and crp. C_LI