Dietary tea tree essential oil addition to a low fishmeal diet containing high-Clostridium autoethanogenum protein diet improves the growth, lipid metabolism, antioxidant capacity and immunity of largemouth bass (Micropterus salmoides)

The study aimed to explore the effects of adding tea tree essential oil (TTEO) to a low fishmeal diet on the growth, lipid metabolism, antioxidant capacity, and immunity of largemouth bass ( Micropterus salmoides ). Three isonitrogenous and isolipidic diets containing high- Clostridium autoethanogenum protein with dietary TTEO (0, 5 g/kg and 10 g/kg, and designated as Control, T5 and T10 groups) were formulated. A total of 360 largemouth basses (initial body weight: 15.06 ± 0.05 g) were randomly distributed into 9 cages and fed these diets for 8 weeks. Compared to the control group, supplementation of 5 g/kg of TTEO could significantly improve the weight gain rate (WGR) and specific growth rate (SGR) ( P < 0.05). The intestinal trypsin activity in the T5 group was significantly higher than in the control group ( P < 0.05). Compared to the control group, the addition of 5 g/kg of dietary TTEO elevated the total protein (TP) and albumin (ALB) contents, and decreased glucose (GLU) content and the total amount of transaminases ( P < 0.05). The triglyceride (TG) contents in serum and crude lipid in the liver of the T5 group were significantly reduced ( P < 0.05). Compared to the control group, TTEO could promote β-oxidation of fatty acid by upregulating the expression of peroxisome proliferator activated receptor alpha ( ppar-α ), carnitine palmitoyl-transferase 1 ( cpt-1 ), and downregulating acetyl-CoA carboxylase beta ( acc-β ) ( P < 0.05). In antioxidative parameters, 5 g/kg of dietary TTEO significantly reduced the contents of malondialdehyde (MDA) and protein carbonyls (PC) in serum, liver and intestine, and increased the total antioxidant capacity (T-AOC) in serum and superoxide dismutase (SOD) activity in serum and intestine compared with the control group ( P < 0.05). Histology analysis observed that 5 g/kg of dietary TTEO ameliorated vacuolization and nuclear excursion of hepatocytes induced by a low fish meal diet. After Aeromonas hydrophila administration, TTEO could upregulate the expression levels of anti-inflammatory-related genes such as il-10 and tgf-β1 compared with the control group ( P < 0.05). In conclusion, 5 g/kg TTEO supplementation in a low fish meal diet could promote growth performance, reduce lipid deposition, enhance liver health, and improve antioxidant capacity and immunity of largemouth bass. • Dietary tea tree essential oil (TTEO) in a low fish meal diet could improve the growth performance of largemouth bass. • Dietary TTEO could improve the antioxidative capacity and immunity through the Keap1-Nrf2 and TLR2-MyD88 pathways. • TTEO also promoted hepatic β-oxidation of fatty acids to reduce lipid deposition.