Intestinal morphology, immunity and microbiota response to dietary fibers in largemouth bass, Micropterus salmoide

This study is aimed at identifying the effects of dietary fiber on gut health, as well as the association between that understanding and fiber consumption in fish. A total of 300 juvenile largemouth bass (micropterus salmoides, initial average weight: 15.38 ± 0.16g) were randomly divided into three treatment groups (4 replicates per group). Fish were fed with isoproteic and isolipidic diets containing 0% (low fiber, LF), 4% (moderate fiber, MF) and 8% (high fiber, HF) soybean fiber, respectively. The intestine and intestinal content of test fish per treatment group after 56 days of treatment were sampled. The results showed that the anterior intestinal sections had normal histological architecture, and no considerable damage or inflammation was observed in any histological section from all subjects examined. Curiously, fish fed the MF diet had better histological alterations than the other treatments. Meanwhile, the intestinal antioxidant capacity in the MF group was significantly promoted when compared to the other groups, as well as up-regulated expression of antioxidant-related genes including sod, cat and gpx with increasing dietary fiber concentrations. Importantly, the administrations of MF diet remarkably elevated largemouth bass innate immune parameters include intestinal inducible nitric oxide synthase (iNOS) activity, nitric oxide (NO) and total protein content. Similarly, dietary administrations of fiber down-regulated notablely the expression of pro-inflammatory cytokines including IL-8, IL-1β and TNFα, whereas up-regulated tolerogenic cytokine IL-10 and TGF-β1 mRNA levels. In addition, dietary fibers also modulated the community structure of the intestinal microbiota by significantly altering bacterial diversity. Dietary supplemental fibers regulated intestinal microbiota in largemouth bass, characterized by a reduced abundance of Fusobacteria along with increased abundances of Proteobacteria and Firmicutes. Taken together, the present results suggested that moderate fiber supplementation was beneficial to promoting intestinal health status of fish through antioxidant and anti-inflammatory effects, which could be at least partially responsible by the modulation of gut microbial composition.