Bacterial infections caused by Edwardsiella tarda and Aeromonas hydrophila pose a serious challenge to sustainable aquaculture. Although antibiotics remain widely used for disease control, the emergence of antimicrobial resistance has stimulated the search for environmentally friendly alternatives. Antimicrobial peptides (AMPs) are promising candidates because they combine direct antibacterial activity with immune-regulatory functions. Scy-Hep3 (EC), a hybrid AMP derived from Scygonadin and Hepcidin3, has shown potent antibacterial activity in vitro; however, it’s in vivo anti-infective efficacy and underlying mechanisms have not been systematically evaluated. To investigate the effects and mechanisms of dietary supplementation with the antimicrobial peptide Scy-Hep3 (EC) on enhancing resistance of marine medaka (Oryzias melastigma) against E. tarda and A. hydrophila infections. Three-month-old marine medaka were randomly divided into three groups: control (C), empty plasmid control (B), and antimicrobial peptide-supplemented group (E), with three replicates each, and fed for 30 days. On days 10, 20, and 30, fish were intraperitoneally injected with pathogens (1×10⁸ CFU/mL), and survival rates were recorded. After 30 days of feeding, fish were challenged and sampled at 24 h post-infection to determine bacterial loads in liver and intestine and to analyze gene expression. E. tarda infection: Feeding EC for ≥20 days significantly improved survival in Group E (P<0.05). After 30 days of feeding, at 9 h post-infection, the survival rate of Group E was 35% and 50% higher than those of Groups C and B, respectively, and bacterial loads in liver and intestine were significantly reduced. In the liver, antioxidant genes (sod, gpx) were significantly upregulated, while cat and il-8 were significantly downregulated. In the intestine, tight junction genes (tjp1b, oclna) and gpx were significantly upregulated. A. hydrophila infection: Feeding EC for ≥10 days significantly improved survival in Group E (P<0.05). After 30 days of feeding, at 9 h post-infection, the survival rates of Groups C and B dropped to 10% and 25%, respectively, whereas Group E maintained 50% survival. Bacterial loads in liver and intestine were significantly reduced in Group E. In the liver, sod, gpx, and hamp2 were significantly upregulated, while cat and il-8 were significantly downregulated. in the intestine, tjp1b, oclna, and c3 were significantly upregulated.

Conclusion Dietary supplementation with Scy-Hep3 (EC) significantly enhances the resistance of marine medaka to E. tarda and A. hydrophila by improving survival, reducing tissue bacterial loads, and regulating the expression of immune and antioxidant-related genes. This study provides a theoretical basis and technical support for the green prevention and control of bacterial diseases in aquaculture.