NMR-based metabolomic investigation of antimicrobial mechanism of electrolysed water combined with moderate heat treatment against Listeria monocytogenes on salmon

Heat and acidic electrolysed water (AEW) are considered effective for inactivation of microorganisms. While a previous study elucidated their bactericidal mechanism on Listeria innocua through metabolomics in vitro, the fundamental mechanism for inaction of L. monocytogenes using this method in food system is poorly understood. This work determined the survival population and injury of three L. monocytogenes strains (SSA81, LM44, and LM3) inoculated on salmon under heat (60 °C) and AEW (100 mg/L available free chlorine, pH 2.42, oxidation reduction potential 1182 mV) treatments, when applied alone or in combination. The bactericidal mechanism was explored by utilising nuclear magnetic resonance (NMR) and multivariate data analysis. Our results indicated that the individual heat and AEW treatment led to 0.4 and 1.2 log CFU/g reductions of L. monocytogenes, respectively. The combined treatment of heat and AEW resulted in notable reductions which were 2.1–2.2 log CFU/g for L. monocytogenes. More than 25%, 35%, and 55% injury of L. monocytogenes were observed under heat, AEW, and the combined treatment, respectively. Overall, 43 metabolites were characterised in three strains. Short time heat might activate protective system of the cells by accumulating amino acids and organic acids. AEW resulted in the reductions of metabolites due to oxidative and acid stress. In the combined treatment, AEW played the main role and its bactericidal ability was strengthened by heat. Significant decreases of Val, Leu, Tyr, and Trp were detected in all strains under the synergic stress of heat and AEW (P < 0.05). There were 15, 7, and 6 pathways, mainly included amino acid, energy, and carbohydrate metabolisms, changed significantly under the combined treatment for SSA81, LM44, and LM3, respectively. The strain LM3 presented the strongest resistance to oxidative stress by the enhancement of Glu decarboxylase system, whereas this compensatory pathway was diminished in SSA81 and LM44. These findings suggest that the bactericidal mechanism can be well explained by disturbed pathways.