Soy protein hydrolysates induce menaquinone-7 biosynthesis by enhancing the biofilm formation of Bacillus subtilis natto

Menaquinone-7 (MK-7) is a form of vitamin K₂ with health-beneficial effects. A novel fermentation strategy based on combining soy protein hydrolysates (SPHs) with biofilm-based fermentation was investigated to enhance menaquinone-7 (MK-7) biosynthesis by Bacillus subtilis natto. Results showed the SPHs increased MK-7 yield by 199.4% in two-stage aeration fermentation as compared to the SP-based medium in submerged fermentation, which was related to the formation of robust biofilm with wrinkles and the enhancement of cell viability. Moreover, there was a significant correlation between key genes related to MK-7 and biofilm synthesis, and the quorum sensing (QS) related genes, Spo0A and SinR, were downregulated by 0.64-fold and 0.39-fold respectively, which promoted biofilm matrix synthesis. Meanwhile, SPHs also enhanced the MK-7 precursor, isoprene side chain, supply, and MK-7 assembly efficiency. Improved fermentation performances of bacterial cells during fermentation were attributed to abundant oligopeptides (Mw < 1 kDa) and moderate amino acids, particularly Arg, Asp, and Phe in SPHs. All these results revealed that SPHs were a potential and superior nitrogen source for MK-7 production by Bacillus subtilis natto.