Studies on Phenylalanine Metabolism in Enhancing the ε-Polylysine-Mediated Killing Effect of Oxytetracycline-resistant Aeromonas hydrophila

AIMS: Aeromonas hydrophila is a zoonotic pathogen, and its prevalence of drug resistance underscores the urgency of elucidating alternative bactericidal strategies. This study employed the oxytetracycline-resistant strain OTC-A01 to investigate the bactericidal mechanism of ε-polylysine (ε-PL). METHODS AND RESULTS: With increasing ε-PL concentration, the survival rate of OTC-A01 decreased to 0.101% at an ε-PL concentration of 100 μg mL-1. Scanning electron microscopy (SEM) and membrane integrity assays confirmed that ε-PL disrupted the bacterial membrane. This damage triggered the leakage of nucleic acids and proteins, increased relative conductivity and membrane permeability, elevated extracellular alkaline phosphatase (AKP) activity, and decreased respiratory chain dehydrogenase activity. Non-targeted metabolomics identified 142 differentially expressed metabolites, with S-plot analysis revealing 74 downregulated and 68 upregulated metabolites. KEGG enrichment indicated phenylalanine metabolism as the most affected pathway, validated by RT-qPCR showing suppressed key enzyme genes (DAADH, CAT, HPPD, and AST). Notably, supplementation with downregulated exogenous metabolites significantly enhanced the bactericidal effect of ε-PL, among which 2-phenylacetamide exerted the most pronounced enhancement (up to 436-fold). Correspondingly, the intracellular concentration of ε-PL reached 50.26 μg mg-1 protein under this condition. CONCLUSION: These results demonstrated that ε-PL exerted its bactericidal effect against strain OTC-A01 mainly through disrupting cell envelope integrity, accompanied by significant downregulation of the phenylalanine metabolism pathway. Notably, supplementation with downregulated metabolites from this pathway, particularly 2-phenylacetamide, significantly enhanced the bactericidal effect of ε-PL, suggesting that metabolic modulation may serve as an adjunctive strategy to potentiate the membrane-directed antibacterial action of ε-PL.