Effect of amino acids on free exopolysaccharide biosynthesis by Streptococcus thermophilus 937 in chemically defined medium

Free exopolysaccharide (f-EPS) produced by Strep-tococcus thermophilus improves the texture and func-tionality of fermented dairy foods. Our previous study showed a major improvement in f-EPS production of Strep. thermophilus 937 by increasing the concentra-tions of histidine, isoleucine, and glutamate to 15 mM in an optimized chemically defined medium. The aim of this study was to elucidate the effect of His, Ile, and Glu on the growth, f-EPS biosynthesis pathway, and carbohydrate metabolism profiles of Strep. thermophi-lus 937. The growth kinetics; transcript levels of key genes in the EPS biosynthesis pathway; enzyme activity involved in sugar nucleotide synthesis; concentrations of lactic acid, lactose, and galactose; and extracellular and intracellular pH were analyzed in chemically de-fined media with different initial histidine, isoleucine, and glutamate concentrations. The results showed that f-EPS production and viable cell counts of Strep. thermophilus 937 increased 2-fold after the concentra-tions of His, Ile, and Glu were increased. Additionally, increasing the concentrations of His, Ile, and Glu up -regulated transcription of EPS biosynthesis genes and increased the activity of key enzymes in sugar nucleo-tide synthesis. Moreover, the consumption of lactose in-creased and secretion of galactose decreased, indicating that increasing the concentration of His, Ile, and Glu could enhance f-EPS production by maintaining viable cell counts, promoting sugar nucleotide synthesis, and increasing the transcript levels of the eps gene cluster. Our results provide a better understanding of the effect of AA on EPS biosynthesis in Strep. thermophilus.

Automated summary

Increasing the concentrations of histidine, isoleucine, and glutamate can significantly enhance the production of free exopolysaccharide (f-EPS) and the viability of Streptococcus thermophilus cells. This increase is achieved by up-regulating the transcription of EPS biosynthesis genes and enhancing the activity of key enzymes involved in sugar nucleotide synthesis. Furthermore, the higher concentrations of histidine, isoleucine, and glutamate result in increased lactose consumption and decreased galactose secretion, indicating the improved production of f-EPS.