Intestinal flora and liver metabolism molecular mechanism on growth and health in Japanese eel black fry (Anguilla japonica) fed with fermented soybean meal replacing soybean meal and fish meal

This study aimed to investigate the effects of fermented soybean meal (FSBM) replacing fish meal (FM) and soybean meal (SBM) on intestinal flora and liver metabolism molecular mechanism in Japanese eel black fry ( Anguilla japonica ), exploring the mechanism on growth and health. Five experimental diets were prepared: the control diet (S5) contained 60 % FM and 5 % SBM, while the experimental diets substituted 11.7 % and 23.3 % of FM in S5 with low concentration of 10 % (S5 +F10) and high concentration of 20 % (S5 +F20) FSBM, respectively. Additionally, 4.5 % of FSBM replaced 100 % of SBM in both S5 +F10 and S5 +F20, treated as two additional experimental diets (F10 and F20). Fish with an initial average body weight of 0.405 ± 0.003 g were fed with above diets for 76 days. The results regarding intestinal flora showed that 10 % FSBM could promote the alpha diversity and community richness of intestinal microflora than control group; In the diets with about 10 % FSBM, 5 % SBM could improve the alpha diversity of intestinal flora than deficiency SBM. A little of FSBM and SBM could positively influenced the community composition of intestinal flora and reduce the abundance of pathogenic bacteria. The combined analysis of liver metabonomic and transcriptomic showed that FSBM significantly changed transcription of the genes which involved citric acid cycle and glyoxylate and dicarboxylate metabolism, and thus altering the level of key metabolites and improving liver metabolic efficiency. At low FSBM concentration, the absence of SBM did indeed cause transcriptional changes in the genes related to purine metabolism, arginine and proline metabolism, including downregulation of genes regulating enzymes rpoA and ADA, upregulation of proA and proB gene expression, upregulating of key metabolites adenine and putrescine. At high FSBM concentration, regardless of the presence of SBM, various mRNA and enzymes exhibited alterations. In conjunction with the previously obtained results on growth and physiology, present findings indicated that incorporating FSBM and SBM into the feed can alter the diversity and richness of microflora, while improving metabolic efficiency of malate and other metabolites, and can ultimately be beneficial to the growth performance, antioxidant capacity, and liver function of A. japonica . • FSBM/SBM modulates intestinal microbiota diversity and richness. • FSBM/SBM enhances metabolic efficiency via MDH1-mediated malate synthesis. • SBM's metabolic effects are dose-dependent on FSBM inclusion levels. • Low-FSBM diets with SBM exclusion upregulate putrescine through proA/proB, impairing growth. • High-FSBM diets induce citric acid cycle and arginine-proline pathway shifts via MDH1/rocD despite comparable growth.