Dietary phytosterols increased the rate of weight gain, antioxidant capacity and growth of beneficial strains of bacteria in the gut and suppressed the population of potentially pathogenic bacteria in largemouth bass (Micropterus salmoides).

This study was to evaluate the effects of the dietary addition of phytosterol (PS) on growth, antioxidant capacity, tissue morphology, and gut microbiota of largemouth bass (Micropterus salmoides). A total of 540 largemouth bass (13.75 ± 0.37 g) were selected and randomly divided into 6 groups and fed with diets supplemented with 0%, 0.01%, 0.02%, 0.03%, 0.04%, and 0.05% of PS (Control, PS1 to PS5) for 56 d. The results showed that the rate of weight gain and condition factor of largemouth bass increased significantly (P < 0.05) with the increasing PS addition levels. In addition, PS significantly increased the activities of superoxide dismutase and catalase in serum compared to group C (P < 0.05), as well as increased relative expression of pituitary Insulin-like growth factor (igf1ra, igf1rb, and igf2r), hypothalamic growth hormone gh and insulin receptor ir, compared with group C (P < 0.05). Histopathological analysis showed that the addition of 0.04% PS to the feed resulted in a tighter cellular arrangement and a significant reduction in vacuolization in the liver of largemouth bass. It also significantly increased intestinal fold width and muscle wall thickness (P < 0.05). Analysis of the intestinal flora showed a significant decrease in the α-diversity of the flora in the PS4 group compared to the C group (P < 0.05). Notably, at the phylum level, the addition of PS to feed significantly reduced the relative abundance of Proteobacteria and Bacteroidota. At the genus level, PS significantly increased the relative abundance of Cetobacterium (P < 0.05). The addition of 0.04% PS to the feed promotes growth, antioxidant capacity, improved tissue morphology, and increased abundances of beneficial gut microbiotas in largemouth bass.