In this study, Bacillus natto NT was evaluated for use as a probiotic supplement in the feeds on the growth performance and the growth-related genes' and microRNAs' (miRNAs) expression in the skeletal muscle of grass carp (Ctenopharyngodon idella). Grass carps (ave. wt. 43.96 AE 0.27 g) were fed diets supplemented with 1.87 9 10 9 (Bn1), 3.73 9 10 9 (Bn2), 5.60 9 10 9 (Bn3), 7.47 9 10 9 (Bn4) and 9.33 9 10 9 (Bn5) B. natto NT cells per 100 g feed for 56 days. The control group (BnC) was not supplemented with the B. natto NT. The fish of Bn3, Bn4 and Bn5 groups displayed better growth performance and lower feed conversion ratio (FCR) than the other groups (P < 0.05). Compared with the fish of BnC group, miR-1a, miR-181a, miR-23a and miR-206 expressions increased (P < 0.05), and the myostatin and myocyte enhancer factor C (MEF2C) mRNAs were downregulated (P < 0.05) in the fish of Bn4 and Bn5 groups. The specific growth rate analysis and apparent expressional regulation of the growth-related miRNAs and genes stimulated by B. natto suggest the potential application of B. natto in improving the growth performance on the grass carps.

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