Effect of Bacillus subtilis and fungal enzymes on immune responses and gut microbiota of masu salmon (Oncorhynchus masou masou)

Abstract

The effects of oral administration of probiotics Bacillus subtilis (BS) and Aspergillus sojae-fermented materials (AFM) supplementation on immune responses and the gut microbiome of masu salmon Oncorhynchus masou masou were evaluated. Masu salmon (44.07 ± 7.1 g) were individually raised on a commercial diet without supplementation (control group), with B. subtilis strain DB9011 at 1.0 × 10⁶ cfu/g-feed (BS group), and with A. sojae-fermented materials supplementation at 0.2 % (AFM group) for 24 days. Lysozyme activity in fish serum was determined using turbidimetric assays. Immune-related gene expression and gut microbiota were analyzed through real-time PCR and metagenomics analysis with a next-generation sequencer, respectively. Both the BS and AFM groups showed higher lysozyme activity but lower serum protein concentrations compared to the control group without significant different. The gut bacterial composition in the BS and AFM groups was dramatically different compared to that in the control group. The alpha diversity of the BS group showed significantly greater richness in terms of Chao1, faith_pb, and shannon_entropy. Conversely, the alpha diversity of the AFM group showed significantly greater richness only on faith_pb. The expression of immune-related genes such as TNF-α and IFN-γ was upregulated in the treatment group compared to the control group. IL1-β was upregulated in the AFM group. Regarding IL1-β, no differences were observed between the control and BS group. Thus, results indicated that oral administration of the BS and AFM modify gut microbiota and stimulated the expression of immune-gene expression.