Effects of Dietary Supplementation with Eucommia ulmoides Extract on Growth Performance, Immune Parameters, and Intestinal Microbiota of Largemouth Bass (Micropterus salmoides)

The largemouth bass (Micropterus salmoides) is an important freshwater aquaculture species in China, but the increasing incidence of disease under intensive production conditions has become a major constraint on its sustainable development. This study evaluated the effects of dietary Eucommia ulmoides extract (ELE) supplementation on growth performance, immune and antioxidant responses, intestinal health, gut microbiota, and resistance to largemouth bass virus (LMBV) in juvenile largemouth bass. During a 56-day feeding trial, fish with an initial body weight of 13.14 ± 0.11 g were fed diets containing 0, 30, 60, 90, 120, or 150 mg ELE/kg, with three replicate nets per treatment. Dietary supplementation with 60–90 mg/kg ELE significantly increased weight gain rate (p < 0.05). ELE also improved selected serum antioxidant and immune indices, with the most pronounced responses generally observed at intermediate supplementation levels. In the intestine, the increased Nrf2 and decreased Keap1 mRNA expression were consistent with the potential involvement of Keap1/Nrf2-related redox regulation. ELE also modulated the expression of inflammation- and apoptosis-related genes and increased the expression of selected tight-junction-related genes, suggesting a potential contribution to intestinal inflammatory regulation and barrier maintenance. In addition, ELE supplementation was associated with shifts in bacterial diversity and community composition, although the responses varied among supplementation levels. Following LMBV challenge, the 90 mg/kg group showed the highest observed protection, accompanied by lower splenic expression of the largemouth bass virus major capsid protein (LMBV-MCP) and proinflammatory gene expression, suggesting reduced viral activity and host inflammatory responses. Overall, dietary supplementation with 60–90 mg/kg ELE elicited the most favorable combined responses and may contribute to the regulation of growth, antioxidant and immune status, and intestinal homeostasis in largemouth bass.