Arginine Decarboxylase Gene (adc) Is Essential for Vibrio anguillarum Virulence and Physiological Phenotypes

<i>Vibrio anguillarum</i> is a major pathogenic bacterium causing vibriosis in aquatic animals, leading to substantial economic losses in the global aquaculture industry. Previous studies have indicated that L-arginine modulates the virulence of the pathogen, but the underlying molecular mechanisms remain elusive. The present study aimed to clarify the regulatory role of L-arginine metabolism in <i>V. anguillarum</i> virulence. We first evaluated the effects of L-arginine and its major metabolites (agmatine, putrescine, spermine) on the hemolytic activity of <i>V. anguillarum</i>. Results showed that L-arginine and its metabolites regulated hemolytic activity in a concentration-dependent biphasic manner, with agmatine exerting the most potent promoting effect. To identify the critical metabolic branch involved, four isogenic mutants were constructed targeting key genes in arginine metabolism (<i>adc</i>, <i>astA</i>, <i>astD</i>). Phenotypic analysis revealed that only the <i>adc</i> deletion mutant (Δ<i>adc</i>) exhibited near-complete loss of hemolytic activity, which was dose-dependently restored by supplementation with agmatine, putrescine, or spermine. Transcriptomic analysis identified 704 significantly differentially expressed genes (DEGs) between Δ<i>adc</i> and WT strains, with downregulated DEGs enriched in virulence-associated pathways. Key hemolysin and secretion system genes were validated to be downregulated in ∆<i>adc</i> by quantitative real-time PCR (qRT-PCR). Additionally, Δ<i>adc</i> displayed attenuated anti-phagocytic ability in Tetrahymena co-culture assays, impaired biofilm formation, and increased susceptibility to multiple classes of antibiotics. Collectively, our findings demonstrate that L-arginine modulates <i>V. anguillarum</i> hemolysis and overall virulence through the <i>adc</i>-mediated agmatine biosynthesis branch. This study fills the knowledge gap in the regulatory mechanism of L-arginine on <i>V. anguillarum</i> virulence and provides a potential target for the control of vibriosis in aquaculture.