Effects of Replacing Dietary Fish Meal by Soybean Meal Co-Fermented Using Bacillus subtilis and Enterococcus faecium on Serum Antioxidant Indices and Gut Microbiota of Crucian Carp Carassius auratus

Abstract

Fermented soybean meal (FSM) is an important feed material that can replace fish meal to solve the shortage of animal protein. To improve the utilization of FSM, we optimized the co-fermentation conditions of soybean meal using Bacillus subtilis and Enterococcus faecium and studied the effects of replacing fish meal with different proportions of FSM on serum antioxidant indices and gut microbiota (GM) composition of crucian carp (Carassius auratus). Our results showed that the co-fermentation of soybean meal was the most effective when the ratio of B. subtilis X-2 and E. faecium X-4 was 2:3, glucose addition was 4.5%, KH2PO4 addition was 0.15%, MgSO4·7H2O addition was 0.1%, anhydrous sodium acetate addition was 0.4%, fermentation time was 120 h, and the solid–water ratio was 1:1. Replacing 40% fish meal with FSM in the feed significantly improved the serum T-AOC, POD, and IgM levels in C. auratus. Although there were significant differences in the midgut and hindgut microbiota structures of C. auratus, the addition of FSM to the feed did not cause significant differences in the GM structure, whether in the midgut or hindgut. Therefore, 40% FSM is the most suitable substitute for fish meal in the feed of C. auratus.