The effect of dietary cecropin AD on intestinal health, immune response and disease resistance of juvenile turbot (Scophthalmus maximus L.).

Cecropin AD (CAD) is a commercial cationic antimicrobial peptide that has been seldom studied in marine fish. This study investigated the effects of dietary CAD on intestinal health, immune response, disease resistance, and growth performance of turbot. A diet using fishmeal and plant protein as the main protein resources was used as the control (crude protein 53%, crude lipid 12%). CAD was supplemented into the control diet at the level of 250, 500, 750, and 1000 mg kg-1 to formulate four experimental diets, C1, C2, C3, and C4, respectively. No significant difference was observed in fish growth performance, feed utilization efficiency and whole-body composition among all groups. Dietary CAD significantly increased the activity of lysozyme and complement component 3 level in both serum and distal intestine (DI), as well as the immunoglobulin M content in DI. The gene expression of immune cytokines such as IFN-γ, IL-1β, and chemokine SmCCL19, and the goblet cell number in DI were also significantly increased by dietary CAD supplementation. Compared with the control group, the microbiota analysis indicated group C4 showed significantly decreased α-diversity, obvious alternation in dominant bacteria composition at phylum level, different clustering, and significantly decreased relative abundance of Lactobacillus. Besides, the relative abundance of Bacteroides was significantly decreased in groups C1, C3, and C4. In addition, the lowest mortality of turbot challenged with Edwardsiella tarda was observed in fish fed diets C2 and C3. In conclusion, moderate levels of CAD in diet of turbot improved the intestinal immune response without disrupting the intestinal bacterial community, and enhanced the disease resistance. However, dietary CAD at 1000 mg kg-1 greatly affected the intestinal bacterial composition and showed potentially inhibitory effects towards Lactobacillus.