In the present study, we demonstrate a novel use for a commercially available glutaminase that can be used as a γ-glutamyltranspeptidase in kokumi seasoning production. Soy protein and gluten were hydrolyzed using a protease isolated from Bacillus licheniformis. The resulting protein hydrolysates were γ-glutamylated with a γ-glutamyltranspeptidase, which is sold as a glutaminase from Bacillus amyloliquefaciens, to produce kokumi seasonings. For γ-glutamylation of soy protein hydrolysate, glutamine was added to the reaction mixture. On the other hand, reaction conditions for enzymatic proteolysis were optimized to liberate glutamine from gluten in large amounts, and the addition of glutamine was not required for γ-glutamylation of gluten hydrolysate. The soy protein and gluten hydrolysates as well as their γ-glutamylated products were subjected to taste evaluation. Soy protein hydrolysates were bitter. Although γ-glutamylation significantly reduced bitterness, the taste was still considered unfavorable. γ-Glutamylated gluten hydrolysate is the most preferable sample and had significantly enhanced thickness, kokumi, and umami tastes, with a moderate increase in saltiness.

Polyamines are essential for biofilm formation in Escherichia coli, but it is still unclear which polyamines are primarily responsible for this phenomenon. To address this issue, we constructed a series of E. coli K-12 strains with mutations in genes required for the synthesis and metabolism of polyamines. Disruption of the spermidine synthase gene (speE) caused a severe defect in biofilm formation. This defect was rescued by the addition of spermidine to the medium, but not by putrescine or cadaverine. A multidrug/spermidine efflux pump membrane subunit (MdtJ)-deficient strain was anticipated to accumulate more spermidine and result in enhanced biofilm formation than the MdtJ+ strain. However, the mdtJ mutation did not affect intracellular spermidine or biofilm concentrations. E. coli has the spermidine acetyltransferase (SpeG) and glutathionylspermidine synthetase/amidase (Gss) to metabolize intracellular spermidine. Under the biofilm-forming condition, not Gss but SpeG plays a major role in decreasing the too high intracellular spermidine concentrations. Additionally, PotFGHI can function as a compensatory importer of spermidine when PotABCD is absent under the biofilm-forming condition. Lastly, we reported here that, in addition to intracellular spermidine, the periplasmic binding protein (PotD) of the spermidine preferential ABC transporter is essential for stimulating biofilm formation. IMPORTANCE Previous reports have speculated on the effect of polyamines on bacterial biofilm formation. However, the regulation of biofilm formation by polyamines in Escherichia coli has not yet been assessed. The identification of polyamines that stimulate biofilm formation is important for developing novel therapies for biofilm-forming pathogens. This study sheds light on biofilm regulation in E. coli. Our findings provide conclusive evidence that only spermidine can stimulate biofilm formation in E. coli cells, but not putrescine and cadaverine. Lastly, ΔpotD inhibits biofilm formation even though the spermidine is synthesized inside the cells from putrescine. Since PotD is significant for biofilm formation and there is no ortholog of PotABCD transporter in humans, PotD could be a target for the development of biofilm inhibitors.