Effects of soybean meal fermentation by Lactobacillus plantarum P8 on growth, immune responses, and intestinal morphology in juvenile turbot (Scophthalmus maximus L.)

In the present study, Lactobacillus plantarum P8 was used to ferment soybean meal (SBM), resulting in a significant reduction in the indigestible oligosaccharides (sucrose, raffinose, and stachyose) and anti-nutritional factors (tannin, trypsin inhibitors, glycinin, and β-conglycinin) in SBM. Nine isonitrogenic and isoenergetic experimental diets were formulated in which SBM or L. plantarum P8 fermented SBM (LPFSBM) replaced fish meal (FM) protein with 0 (control); 15% by SBM (Diet 1), 30% by SBM (Diet 2), 45% by SBM (Diet 3), 60% by SBM (Diet 4), 15% by LPFSBM (Diet 5), 30% by LPFSBM (Diet 6), 45% by LPFSBM (Diet 7), and 60% by LPFSBM (Diet 8). Triplicate groups of 30 fish were fed to apparent satiation twice daily for 66 d in an indoor recycling seawater system. No significant differences were found in survival rate among the different groups. However, 45% SBM, 60% SBM, and 60% LPFSBM (Diets 3, 4, and 8) diets significantly reduced growth performance and feed utilization. The LPFSBM diets did not affect moisture, crude protein, crude lipid, or crude ash contents of whole-body measurements. However, 60% SBM (Diet 4) significantly decreased crude protein and lipid content while increasing moisture and crude ash contents of the whole-body. The activity of serum lysozyme was significantly decreased in the group fed with 60% SBM (Diet 4). Total antioxidant capacity was significantly increased in the group fed the 45% LPFSBM (Diet 7). Apparent digestibility coefficient (ADC) of protein was significantly lower in fish fed diets with 45% and 60% of protein from SBM and 60% of protein from LPFSBM than in the control group. When the substitution level of SBM was equal to or above 45%, the activities of protein metabolism enzymes (alanine aminotransferase, ALT and aspartate aminotransferase, AST) were significantly lower than those in the control group. The level of LPFSBM did not affect the activities of ALT and AST. SBM-induced pathological changes seen in the distal intestine of fish fed 45% and 60% SBM (Diets 3 and 4) were not pronounced in groups fed LPFSBM. The results of the present study suggest that protein from SBM could substitute up to 30% of FM protein, with protein from LPFSBM being able to replace up to 45% of FM protein. Statement of relevance The present study was conducted to investigate the effects of Lactobacillus plantarum P8 fermentation on soybean meal and evaluate LPFSBM as a partial replacement for fish meal in diets of turbot by examining growth, activities of protein metabolism enzymes, intestinal morphology, and immune responses. Our results would be helpful to develop cost effective and sustainable dietary formulations for turbot. The results of this study indicate that LPFSBM could replace up to 45% of fish meal protein in juvenile turbot diet. The ANFs are important factors that affect the growth performance of turbot, damaging the normal structure of intestine and a depression of digestion and absorption function. LPFSBM reduced SBM-induced pathomorphological changes in the distal intestine which partly accounted for the L. plantarum P8 fermentation reduces indigestible oligosaccharides and anti-nutritional factors in soybean meal and enhanced the apparent digestibility coefficients (ADCs) of dry matter and crude protein of soybean meal. The results are reliable and of both theoretical and practical importance. The work described has not been submitted elsewhere for publication, in whole or in part, and all the authors listed have approved the manuscript that is enclosed. I have read and have abided by the statement of ethical standards for manuscript submitted to Aquaculture.