Interactions between intestinal morphology, digestion, inflammatory responses, and gut microbiota of juvenile channel catfish elicited by dietary enzymatic rice protein

The reduction of fishmeal in aquafeeds has been the concern of researchers. Replacing fishmeal with plant proteins affects intestinal function and inflammation, but the interaction between the intestinal responses and gut microbiota remains unclear. In this study, juvenile channel catfish (Ictalurus punctatus) was fed with four diets in which enzymatic rice protein (RP) replaced fishmeal at levels of 0 (FM), 2.5% (RP2.5), 5.0% (RP5.0), and 7.5% (RP7.5) for 8 weeks to solve the problem mentioned above. Quantification of intestinal morphology showed that 2.5% or 5.0% RP significantly increased villus length and goblet cell number, accompanied by higher activities of intestinal trypsin, alkaline phosphatase (AKP), and Na+/K+-ATPase (NKA) in RP2.5 group (P < 0.05). In contrast, 7.5% RP slightly damaged the intestinal mucosa and significantly reduced the activities of amylase, AKP, and NKA, as well as decreased serum complement 4 (C4) and immunoglobulin M (IgM). Noteworthy, RT-qPCR showed that 2.5% RP significantly down-regulated intestinal mRNA expression level of il8, while up-regulated mif, tlr4, tlr7, tgfβ3, and cldn2. In contrast, 7.5% RP up-regulated the mRNA expression levels of il1β, il8, and mif, while down-regulated cldn3d. Analysis of gut microbiota showed that 2.5% RP increased the relative abundance of Bacteroidetes and significantly activated potential functions of gut microbiota involved in carbohydrate metabolism. The 7.5% RP increased the diversity of the gut microbiota, accompanied by a significant increase in the relative abundance of conditionally pathogenic bacteria such as Vibrio, Serratia, and Aeromonas (classified as Proteobacteria). Notably, Vibrio was the biomarker species with the greatest difference between the FM and RP7.5 groups (genus level). Correlation analysis indicated that Vibrio may affect immunity through the C4 pathway and further lead to gut inflammation and digestive impairment. Taken above, these results indicated that RP could affect intestinal morphology, digestion, and inflammation, and interact with the composition and potential function of gut microbiota. The low RP supplement (2.5%) improved intestinal morphology and digestion, while high supplement (7.5%) disrupted gut microbiota homeostasis, resulting in damage to intestinal mucosa and inflammatory response.