NMR-based metabolomics reveals the antibacterial effect of electrolysed water combined with citric acid on Aeromonas spp. in barramundi (Lates calcarifer) fillets

Abstract

The citric acid (CA) and electrolysed water (EW) are considered effectively in inactivating microorganisms. The objective of this study was to explore the bactericidal mechanism of CA combined with EW on Aeromonas spp. in barramundi (Lates calcarifer) by in vitro metabolomics method. This study determined the survival population of three strains of Aeromonas bacteria (strain 1: Aeromonas salmonicida strain A1 (skin); strain 2: A. veronii strain Til2 (gut), and strain 3: A. hydrophila strain B11 (gill)), which were isolated and identified from putrid barramundi treated alone or in combination with 1 % CA and EW (free available chlorine (FAC) 25 mg/L, pH 3.23, oxidation-reduction potential (ORP) 1015 mV). The bactericidal mechanism was investigated by microbiological analysis, nuclear magnetic resonance (NMR), multivariate data analysis, and fluorescence staining analysis. The results showed that the combined treatment significantly reduced the number of Aeromonas bacteria at 1.64–1.69 log CFU/g and extended the shelf life of barramundi fillets. In addition, the combined treatment had a higher effect on the cell membrane integrity of the bacteria. In total, 36 metabolites were identified in the three strains. The undissociated molecules of CA can enter the cytoplasm, resulting in cell damage and inhibiting metabolic pathways. EW could lead to the reduction of metabolic products caused by oxidative stress and acid stress. Under the synergistic stress of CA and EW, the changes of main metabolite contents in the combined treatment group were significantly reduced. After combined treatment, there were 20, 31, and 31 pathways in which carbohydrate metabolism, amino acid metabolism, and energy metabolism were changed considerably. These findings indicated that the bactericidal mechanism of the bactericidal substance might be explained by the interference of the metabolic pathway, which guided post-treatment sanitisation and extended the applicability of the NMR spectrum to specific spoilage organisms (SSO) analysis in fish.