Reduced efficacy of food-chain biocides and preservatives against ciprofloxacin-resistant Salmonella Typhimurium.

A wide range of microbial prevention and control strategies are applied along the agri-food chain to combat the transmission of foodborne pathogens to the final consumer. These strategies range from traditional thermal treatments to emerging technologies as well as the use of food chain biocides to reduce microbial loads on raw materials and food contact surfaces, and preservatives for the preservation of feed and food products. However, bacteria have demonstrated a remarkable ability to adapt to and overcome these food-related stresses, often resulting in reduced susceptibility to other seemingly unrelated agents through cross-protection mechanisms. In this study, we aimed to evaluate the outcomes of ciprofloxacin (CIP) antimicrobial resistance (AMR) in Salmonella Typhimurium and its potential relationship with cross-protection to food-chain biocides and preservatives, as well as the ability of the resistant variants (RVs) to retain their virulence. The CIP-RVs showed heterogeneous and compound-dependent responses to the tested food-chain antimicrobial compounds. All five RVs displayed increased tolerance to at least one compound, but cross-protection was not uniform across strains or treatments. Notably, SeTRV1 displayed reduced susceptibility to four of the six agents tested, and derivative strains carrying SeTRV1-derived alleles supported an important contribution of ramR, cyaA, and especially rpoD to this broad phenotype. Moreover, several RVs displayed Galleria mellonella pathogenicity comparable to that of the parental strain, despite occasional reductions or dose-dependent effects, suggesting that reduced susceptibility to food-chain antimicrobial compounds may occur without a major loss of virulence-associated traits in this model. KEY POINTS: • CIP-resistant S. Typhimurium variants can show reduced chemical susceptibility • Cross-protection is associated with global regulation and stress response mutations • CIP-RVs showed divergent pathogenicity patterns in the Galleria model.