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This study was conducted to investigate the effects of tea tree essential oil (TTO) supplementation in low fish meal diet on growth, lipid metabolism, anti-oxidant capacity and immunity of largemouth bass (Micropterus salmoides). Five low fish meal (175 g kg−1) diets with grade levels of TTO (0, 0.25, 0.5, 1 and 2 g kg−1) were formulated to feed largemouth bass (initial weight: 15.06 ± 0.05 g) for 56 days. The results indicated that supplementation of dietary 1 g kg−1 TTO could significantly improve the weight gain rate, specific growth rate and protein deposition ratio, and decrease the feed conversion rate. The activities of gastric lipase and intestinal trypsin were enhanced by dietary TTO. The width of the intestinal villus was significantly elevated with dietary TTO level. In lipid metabolism parameters, the whole-body crude lipid content was increased and the liver crude lipid content was decreased in TTO treatment groups. The expression of acetyl-CoA carboxylase beta was downregulated, and the mRNA levels of peroxisome proliferator activated receptor alpha and carnitine palmitoyl-transferase 1 were upregulated in the liver by the addition of TTO. The contents of total cholesterol, triglyceride and low-density lipoprotein cholesterol significantly decreased, and the content of high-density lipoprotein cholesterol significantly increased in serum by dietary TTO. In anti-oxidative parameters, catalase activities in the liver and superoxide dismutase activities in the liver, intestine and serum were elevated with dietary TTO level. Malondialdehyde and protein carbonyl contents in the liver, intestine and serum showed the opposite trend. The nf-e2-related factor 2 (Nrf2) signaling pathways in the liver and intestine were triggered by dietary TTO supplementation. In the immune indices, the contents of total protein and albumin were improved, and the contents of glucose, aspartate aminotransferase and alanine transaminase were declined in TTO treatment groups. However, hepatocyte swelling and nuclear migration were found in the liver sections of all groups. After Aeromonas hydrophila administration, the toll-like receptor 2 (TLR2) signaling pathway was activated in the head-kidney, liver and intestine, and the expression levels of interleukin 10 and transforming growth factor beta 1 were increased in the head-kidney, spleen, liver and intestine by dietary TTO. In conclusion, dietary TTO in low fish meal diet could improve growth, anti-oxidant capacity and immunity, and reduce lipid deposition in the liver and serum of largemouth bass. The optimal level of TTO in low fish meal diet of largemouth bass ranged from 1.33 to 1.34 g kg−1. |
