Lactobacillus solid-state fermentation with rice bran modulates substrate availability and microbial accessibility of...

Substrate availability and microbial accessibility are critical determinants in the conversion of okara into animal feed through solid-state fermentation (SSF). This study comprised two experiments to (1) optimize Lactobacillus SSF of okara using rice bran for moisture adjustment and (2) evaluate the feasibility of the resulting okara and rice bran fermented product (ORFP) for broiler feeding. Both experiments were interpreted using microbiota and metabolomic analyses. In Experiment 1, okara and rice bran were inoculated with three homofermentative Lactobacillus species (Lactobacillus acidophilus, L. delbrueckii, and L. salivarius) and fermented at four moisture levels (45, 50, 55, and 60%) for 0, 48, 60, and 72 h. The optimal SSF condition was identified at 60% moisture and 72 h, as indicated by increased lactic acid production and degradation of major soybean storage proteins. Fermentation reduced the abundance of Proteobacteria-associated genera (Pantoea, Pseudomonas, and Enterobacter) and increased Lactobacillus and Streptococcus, accompanied by changes in metabolites related to nucleotide, amino acid, and plant-derived compound metabolism. In Experiment 2, 112 one-day-old Arbor Acres broilers were assigned to a basal diet or diets containing 1.25, 2.5, or 5.0% ORFP. Broilers fed 2.5% ORFP maintained feed conversion ratio and increased ileal villus height. Ileal microbiota analysis revealed an increased abundance of the carbohydrate-utilizing bacterium Romboutsia timonensis in the 1.25 and 2.5% ORFP groups. Functional prediction indicated enhanced substrate utilization at these inclusion levels, with broader metabolic enrichment observed at 2.5% ORFP, whereas fewer functional pathways were observed in the 5.0% ORFP group. Further metabolomic analysis indicated enrichment of amino acid- and sulfur-related pathways in the 1.25 and 2.5% ORFP groups. These changes were associated with mucosal-associated metabolism, as shown by the positive association between R. timonensis and glucosamine-6-sulfate. Overall, these results indicate that Lactobacillus SSF combined with rice bran modulates substrate availability and microbial accessibility of okara, with 2.5% ORFP associated with improved intestinal morphology and changes in microbiota and metabolite profiles in broilers.