The Synergistic Antibacterial Activity of Curcumin and Quercetin Against Nocardia seriolae and Evaluate Their Therapeutic Effects in the Infected Largemouth Bass

ABSTRACT Nocardia seriolae is an important bacterial pathogen causing chronic granulomatous disease in cultured fish, and effective therapeutic strategies remain limited. This study aimed to identify natural compounds with synergistic antibacterial activity against N. seriolae and evaluate their protective effects in largemouth bass ( Micropterus salmoides ). A structure‐guided screening strategy targeting N. seriolae isocitrate lyase (ICL) was performed using molecular docking, ADMET prediction, and in vitro antibacterial assays. Curcumin and quercetin were selected as candidate compounds based on their predicted interactions with ICL and antibacterial activity. The combination of curcumin and quercetin exhibited a synergistic effect against N. seriolae , with a fractional inhibitory concentration index (FICI) of 0.25 at a fixed 1:1 ratio, reducing the effective concentration of each compound eightfold compared with individual treatments. Molecular dynamics simulations demonstrated stable interactions between the two compounds and the ICL model, while enzyme activity analysis showed that the combination significantly reduced intracellular ICL activity. Furthermore, the combined treatment inhibited bacterial growth, decreased metabolic activity, and caused severe disruption of bacterial cell envelope integrity, as demonstrated by transmission electron microscopy and alkaline phosphatase leakage assays. Acute toxicity evaluation showed that the 96‐h LC 50 of the curcumin‐quercetin mixture in largemouth bass was 24 mg/L, indicating relatively low short‐term toxicity at the therapeutic concentration. In an experimental infection model, immersion treatment with the curcumin‐quercetin combination reduced bacterial burden, alleviated granulomatous lesions in visceral organs, and improved survival of infected fish. Collectively, these findings demonstrate that curcumin and quercetin provide a promising natural‐compound combination for controlling N. seriolae infection and provide a potential strategy for managing fish nocardiosis.