Administration of low-dose dietary hydrolysable tannins exhibited enhanced growth, antioxidant capacity, and modulation of intestinal microbiota in largemouth bass (Micropterus salmoides)

Antinutritional factors (ANFs) such as hydrolysable tannin (HT) are major constraint on the utilization of rapeseed meal. In order to investigate the impact of HT on the growth, antioxidant capacity, digestive enzyme activities and intestinal flora of largemouth bass (Micropterus salmoides), a total of 945 fish with initial weight of (26.15±3.59) g were selected and randomly divided into 7 groups with 3 parallel in each group. And each group were fed isonitrogenous and isoenergetic diets containing 0% (Control group), 0.1%, 0.2%, 0.3%, 0.6%, 0.9% and 1.2% HT (control, HT0.1, HT0.2, HT0.3, HT0.6, HT0.9 and HT1.2) for 8 weeks, respectively. The experimental results showed that compared with the control group, the weight gain rate of HT0.2 and feed coefficient ratio of HT1.2 were significantly increased, the muscle crude protein content was significantly decreased in both HT0.9 and HT1.2, whereas the total lipid content was significantly increased. The total antioxidant capacity (T-AOC) along with the activities of superoxide dismutase (SOD) and glutathione S-transferase (GST) were significantly increased in HT0.2, meanwhile the content of malondialdehyde (MDA) exhibited an increase in HT0.2-HT1.2. Liver T-AOC and the activity of SOD were notably higher in HT0.1 than control group; however, the catalase (CAT) activity decreased gradually with the increase of HT content; MDA content was significantly higher in HT0.6, HT0.9 and HT1.2 than control group. Compared with the control group, T-AOC was significantly decreased in hindgut of tannin groups, CAT and SOD activities in HT0.2 and MDA content in HT0.2-HT1.2 were significantly increased. Furthermore, the activities of pepsin in control group and gastric lipase in HT0.9 were significantly higher than other groups. The activity of foregut amylase initially decreased and then increased with rising level of HT, on the other hand trypsin activity showed an opposite changing trend. There was no significant difference in the α diversity index of intestinal flora among groups, however, an increase in HT content resulted in a trend of initially increasing and then decreasing relative abundance of some beneficial bacteria. A dietary inclusion of 0.2% HT was found to improve the internal environment homeostasis of intestinal flora of M. salmoides. These findings indicated that the growth and health of M. salmoides could be improved by incorporating less than 0.2% HT in the diet, thereby providing valuable insights for the application of HT and plant-based ingredients containing HT, such as rapeseed meal, in aquatic feed.