Specificity of Amino Acid Profiles Produced in Soybean Fermentations by Three Bacillus spp

We compared the salt tolerance and proteolytic activity of 120 strains of each of Bacillus subtilis, Bacillus velezensis, and Bacillus licheniformis. Most B. subtilis strains exhibited growth in 12% (w/v) NaCl and showed proteolytic activity in 10% or 11% NaCl. The majority of B. velezensis strains grew in 14% NaCl and showed proteolytic activity in 12% or 13% NaCl. Most B. licheniformis strains grew in 14% NaCl and exhibited proteolytic activity in 5%-7% NaCl. We selected nine representative strains of each species based on their proteolytic activities and analyzed the free amino acid (FAA) profiles produced by culture of the bacteria on soybean. Statistical analyses of the 22 FAAs quantified in the cultures revealed clustering of FAA production profiles at the species level. The FAA production profiles of B. subtilis and B. velezensis were similar, and both differed from that of B. licheniformis. These trends persisted in cultures containing 7% NaCl. These results suggest that FAA production profiles are characteristic of each Bacillus species. Specifically, in soybean cultures compared with uninoculated soybeans, B. subtilis increased the amounts of leucine and phenylalanine; B. velezensis increased the amounts of leucine, phenylalanine, and tyrosine; and B. licheniformis increased the amounts of alanine, glutamic acid, tyrosine, and ornithine, and dramatically decreased the amount of arginine. The proteolytic activity of B. velezensis strains correlated with the quantity of FAAs in their soybean cultures. Considering its salt tolerance and proteolytic activity, B. velezensis showed high potential for contributing to the ripening of high-salt fermented soybean foods. Our results regarding the specific production of amino acids at the species level and correlations between proteolytic activities and produced amino acid quantities will facilitate the determination and selection of target strains for functional Bacillus-fermented foods.