Macleaya cordata Alkaloids Sanguinarine and Chelerythrine Inhibit Nocardia seriolae by Disrupting Cell Envelope Integrity and Energy Metabolism: Insights from Transcriptomic Analysis

<i>Nocardia seriolae</i> infection poses a serious threat to largemouth bass (<i>Micropterus salmoides</i>) aquaculture, owing to the lack of effective control strategies. This study investigated the antibacterial effects and underlying mechanisms of <i>Macleaya cordata</i> alkaloids-sanguinarine (SE) and chelerythrine (CHE)-against <i>N</i>. <i>seriolae</i> through integrated physiological and transcriptomic approaches. Results showed SE and CHE exhibited strong in vitro antibacterial activity, with minimum inhibitory concentrations (MICs) of 62.5 and 7.8 μg/mL, respectively. In vivo trials revealed that dietary supplementation with either alkaloid significantly enhanced the survival of infected fish, yielding relative percent survival (RPS) values of 34.5% for SE and 40.0% for CHE. Concurrently, both treatments reduced bacterial load and alleviated granulomatous pathology in multiple organs, including the liver, spleen, and kidney. Physiological analyses revealed severe damage to the cell envelope, as evidenced by increased membrane permeability and structural disintegration observed under transmission electron microscopy (TEM). Transcriptomic profiling identified 3708 and 5095 differentially expressed genes (DEGs) in the SE- and CHE-treated groups, respectively, with notable downregulation of key genes involved in peptidoglycan biosynthesis, the citrate cycle, oxidative phosphorylation, and the pentose phosphate pathways. These findings demonstrate that SE and CHE inhibit <i>N. seriolae</i> through a multi-target mechanism simultaneously disrupting cell envelope integrity and energy production, laying the groundwork for their development as eco-friendly aquaculture therapeutics.