High dietary starch impairs intestinal health and microbiota of largemouth bass, Micropterus salmoides

This study focused on the effects of starch levels on growth performance, intestinal dysbiosis and health of Micropterus salmoides. Three isonitrogenous and isolipidic diets containing different corn starch levels (5%, 10%, 15%) were fed to experimental fish (initial body weight: 8.66 +/- 0.04g) for 8 weeks, respectively. The results showed that the 15% starch levels suppressed the growth, decreased the height and width of villus and number of goblet cell in midgut compared with the other groups (P < 0.05). Moreover, plasma endotoxin (PE) and diamine oxidase (DAO) increased significantly with dietary starch levels (P < 0.05). Notably, 15% starch diets upregulated the expression of Claudin-1 and down-regulated the expression of ZO-1, Claudin-4 and Claudin-5 in intestine (P < 0.05). Meanwhile, fish fed 15% starch diets decreased antioxidant enzyme mRNA levels (Nrf2, CAT and SOD) and activities (SOD and CAT) compared to the other groups (P < 0.05). The increased dietary starch levels upregulated the expression of pro-inflammatory cytokines (IL-8, IL-1 beta and TNF alpha) and anti-inflammatory cytokines (IL-10 and TGF-beta 1) (P < 0.05). Additionally, the abundance of intestinal probiotics (such as Lactobacillus) decreased with the increase of dietary starch level (P < 0.05), while abundance of intestinal potentially pathogenic bacteria (such as Brevundimonas and Ralstonia) increased (P < 0.05). Fish fed 15% starch diet had less concentrations of acetate and butyrate in intestinal digesta than the 5% starch group (P < 0.05). These results suggested that 15% starch diets would impair the intestinal health, suppress immune and antioxidant capacity and finally reduced growth of Micropterus salmoides. The imbalances in intestinal microbiota and the reduced production of short-chain fatty acids (SCFA) may be partly responsible for the loss of intestinal function.