Effects of Nutrients on the Growth of and Free Exopolysaccharide Biosynthesis by Streptococcus thermophilus 937 in a Chemically Defined Medium.The free exopolysaccharide (f-EPS) produced by Streptococcus thermophilus is a natural texture modifier with health-promoting properties and has thus become one of the most interesting metabolites for researchers.  The present work aimed to further understand the nutritional requirements for the growth of and the f-EPS production by S. thermophilus.  The types and concentrations of compounds in the complete chemically defined medium were changed in turn to evaluate the effects of single nutrients on the growth of and f-EPS production by S. thermophilus 937.  The results showed that cysteine, glutamine, histidine, methionine, tryptophan, tyrosine, leucine, isoleucine, and valine played an important role in maintaining the rapid and stable growth of S. thermophilus 937.  S. thermophilus 937 also required calcium pantothenate, niacin, pyridoxine, riboflavin, and thiamine hydrochloride as essential nutrients for growth.  Increases in the concentrations of lactose, glutamate, histidine, or isoleucine significantly increased the production of free exopolysaccharide by S. thermophilus 937, and when the lactose concentration increased to 20 g·L-1 and the concentration of the three-amino-acid combination increased to 15 mM, the f-EPS yield increased to a maximum of 35.34 μg·mL-1.  In addition, lactose and the three amino acids have strain specific promoting effects on f-EPS production by S. thermophilus.  This study provides a further understanding of the effects of nutrients on the biosynthesis of f-EPS by S. thermophilus.
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