Abstract
This study investigated the effects of crowding stress on liver and gut health in genetically improved farmed tilapia (GIFT, Oreochromis niloticus). Fish averaging 18.90 ± 0.01 g were reared at low (LD, 5 g/L) or high densities (HD, 100 g/L) for 14 days. The analysis revealed that the HD group significantly increased serum cortisol, glucose and adrenocorticotropic hormone (ACTH) levels in GIFT (P < 0.05). Serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels were elevated in the HD group (P < 0.05). TUNEL staining of liver tissues revealed that the fluorescence intensity indicating apoptosis was significantly higher in the HD group (P < 0.05). Additionally, the HD group exhibited increased expression of p53 and Caspase3 in the liver (P < 0.05). In comparison to the LD group, the HD group showed a significant increase in gut superoxide dismutase (SOD) and catalase (CAT) activities, as well as malondialdehyde (MDA) content (P < 0.05). The expressions of inflammatory-related genes (tnfα, il-1β, il-8, and il-6) and pro-apoptotic genes (p53 and Caspase7) were significantly upregulated in the gut of the HD group (P < 0.05). Serum contents of D-lactate and diamine oxidase (DAO) were significantly elevated in HD group as comparison to LD group (P < 0.05), and significantly higher serum FITC-CM-dextran was observed in HD group versus LD group following oral gavage (P < 0.05). Furthermore, the HD group down-regulated the expressions of gut tight junction proteins (tjp2a, hif-1a, and zo-1) (P < 0.05), and significantly up-regulated the protein expression of phosphorylated myosin light chain 2 (P-MLC2) (P < 0.05). The 16S rDNA gene sequencing results indicated that the relative abundance of Cetobacterium was significantly lower (P < 0.05), and Enterovibrio and Weissella were significantly higher in the HD group (P < 0.05). Overall, crowding stress negatively affects GIFT liver and gut health, which is caused by inducing hepatic and intestinal apoptosis, impairing intestinal barrier function, and disrupting the gut microbiota.
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