The substitution of fish meal-soybean meal mixture with fermented soybean meal promoted the growth of bullfrogs (Lithobates catesbeianus) to the same performance as florfenicol addition through positively affecting intestinal health and microbiota

The aim of this experiment was to investigate the effect of fermented soybean meal (FSBM) replacing fish meal (FM) + soybean meal (SBM) on growth, serum biochemical indices, liver and intestinal histology, intestinal digestive enzyme activities and microbial composition of bullfrogs (Lithobates catesbeianus). The basal diet contained 100.0 g/kg of FM and 290.0 g/kg of SBM, then 40.0 g/kg, 80.0 g/kg of FSBM was used to replace the same amount of FM-SBM mixture (1:3) (FSBM40, FSBM80). In addition, 2.0 g/kg florfenicol (10 % purity) was added into basal diet to form the antibiotic diet (ANTI). The above four isonitrogenous diets were fed to bullfrogs with initial body weight of 45.5 ± 0.4 g for 50 days. The FSBM80 and ANTI groups presented significantly higher weight gain and lower feed conversation ratio than the control group (P < 0.05), while no significant difference was detected among the three groups of FSBM40, FSBM80 and ANTI (P > 0.05). The ANTI group also showed significantly higher condition factor than the control group and significantly higher mesenteric fat index than the other three groups (P < 0.05). The florfenicol content in the muscle of ANTI group was 19.9 μg/kg, but it was not detected in the other three groups. The serum lysozyme activity, liver total superoxide dismutase, catalase activities, total antioxidant capacity, intestinal muscle thickness were significantly increased, and serum aspartate aminotransferase, alanine aminotransferase activities and liver malondialdehyde content were significantly decreased in the FSBM80 group when compared to the control group (P < 0.05). Trypsin, α-amylase activity and lipase activity were significantly higher in FSBM80 and ANTI groups than those in the control group (P < 0.05). Compared with the control group, the ANTI group presented significantly higher malondialdehyde content in liver and incomplete hepatocyte structure (P < 0.05). In terms of intestinal microorganisms, the FSBM80 group revealed higher indices of sobs, shannon, ace, chao, higher abundance of Fusobacteriota, Proteobacteria and lower abundance of Bacteroidota and Mycoplasma than the control group, while the ANTI group showed significantly lower sobs, shannon and chao indices than the FSBM80 group, and lower Ace index than the control group (P < 0.05). In conclusion, the replacement of 80 g/kg FM-SBM mixture (1,3) with 80 g/kg FSBM promoted the growth performance of bullfrogs to the same level as the antibiotic group, enhanced the intestinal digestive enzymes activity, and positively regulated the intestinal microorganism community.