Effects of dietary protein levels with cottonseed protein concentrate inclusion on growth, feed utilization, liver health and intestinal microbiota of juvenile largemouth bass (Micropterus salmoides)

An 8-weeks growth trial was conducted to evaluate the effects of dietary protein levels mainly based on cottonseed protein concentrate (CPC) meal on growth performance, protein metabolism, liver health and intestinal microbiota of largemouth bass (Micropterus salmoides) juveniles. Five isolipidic (100 g/kg) diets were prepared with protein levels ranging from 400 to 560 g/kg, with CPC of 155 g/kg, 214 g/kg, 272 g/kg, 330 g/kg, 390 g/kg to obtain five experimental diets with gradient protein levels (399.9 g/kg, 428.0 g/kg, 462.8 g/kg, 503.5 g/kg and 541.5 g/kg, respectively (CPC1, CPC2, CPC3, CPC4 and CPC5)). 300 fish (initial body weight: 14.71 +/- 0.50 g) were randomly assigned into 15 tanks (3 tanks per diet). The results showed that, the fish of CPC5 group had the highest specific growth rate (SGR), weight gain (WG) and feed efficiency (FE) (P < 0.05). Increased dietary protein up-regulated most liver key genes of mTOR pathway (akt, tor, s6k, 4ebp1, s6) while down-regulated atf4 at medium dietary protein (P < 0.05). The expression of fas was down-regulated with increased dietary protein, while dgat1 was higher at CPC5 group (P < 0.05). The genes related to lipolysis were up-regulated with increased dietary protein (P < 0.05). The key genes of apoptosis including bcl2, casp9, casp3 (apoptosis) oxidative stress and endoplasmic reticulum stress were significantly up-regulated at high dietary protein (P < 0.05), which reduced the antioxidant and anti-stress capacity of the liver. Dietary 462.8 g/kg protein could maintain the normal composition and metabolic function of intestinal microbiota. In summary, dietary protein requirement using CPC as main protein source was estimated to be 463 g/kg to 510 g/kg for liver health or best growth of largemouth bass juveniles.