Inactivated lactobacillus plantarum promoted growth performance, intestine health and antioxidant capacity of juvenile largemouth bass, Micropterus salmoides

To evaluate the effects of inactivated Lactobacillus plantarum (LPM) on carnivorous fish, juvenile largemouth bass [initial body mass: (5.62 +/- 0.13) g] were fed five isonitrogenous and isoenergetic diets containing 0 (M0), 200 (M1), 400 (M2), 600 (M3), and 800 (M4) mg/kg LPM for 8 weeks. The results showed that in terms of growth, weight gain rate (WGR) and specific growth rate (SGR) showed significant second-order polynomial trend with increasing LPM supplementation. WGR and SGR of the M3 group were significantly increased by 14.18% and 5.62%, respectively, compared with the M0 group. Significant linear and second order polynomial trends were observed for feed conversion ratio (FCR), protein efficiency ratio (PER) and protein deposition rate (PDR) with the increase in dietary LPM. FCR was significantly reduced by 8.9% in the M2 group compared to the M0 group. In terms of intestinal health, intestinal amylase, intestinal villi number, length and width showed significant second order polynomial trends with increasing LPM supplementation. Significant linear and second order polynomial trends were observed for intestinal lipase activity, liver crude fat content with increasing dietary LPM. Dietary LPM supplementation of 600 mg/kg significantly increased the abundance of beneficial bacteria. In terms of antioxidant capacity, with the increase of LPM supplemental level, superoxide dismutase activity, malondialdehyde content of serum and liver, and liver Cu/Zn-SOD, Keap-1 and TGF-beta 1 mRNA expression levels showed significant linear and second-order polynomial trend, and Nrf2 mRNA expression level showed a significant second-order polynomial trend. Overall, the appropriate level of LPM (400-600 mg/kg) promoted the growth performance and feed utilization of largemouth bass by increasing the abundance of intestinal bacteria, improving the activities of digestive enzymes and antioxidant enzymes, and up-regulating the mRNA expressions of genes related to antioxidants.