Correlation of activities of the enzymes alpha-phosphoglucomutase, UDP-galactose 4-epimerase, and UDP-glucose pyrophosphorylase with exopolysaccharide biosynthesis by Streptococcus thermophilus LY03

The effects of different carbohydrates or mixtures of carbohydrates as subs trates on bacterial growth and exopolysaccharide (EPS) production were stud ied for the yoghurt starter culture Streptococcus thermophilus LY03. This s train produces two heteropolysaccharides with the same monomeric compositio n (galactose and glucose in the ratio 4:1.) but with different molecular ma sses. Lactose and glucose were fermented by S. thermophilus LY03 only when they were used as sole energy and carbohydrate sources. Fructose was also f ermented when it was applied in combination with lactose or glucose. Both t he amount of EPS produced and the carbohydrate source consumption rates wer e clearly influenced by the type of energy and carbohydrate source used, wh ile the EPS monomeric composition remained constant (galactose-glucose, 4:1 ) under all circumstances. A combination of lactose and glucose resulted in the largest amounts of EPS. Measurements of the activities of enzymes invo lved in EPS biosynthesis, and of those involved in sugar nucleotide biosynt hesis and the Embden-Meyerhof-Parnas pathway, demonstrated that the levels of activity of alpha-phosphoglucomutase, UDP-galactose 4-epimerase, and UDP -glucose pyrophosphorylase are highly correlated with the amount of EPS pro duced. Furthermore, a weaker relationship or no relationship between the am ounts of EPS and the enzymes involved in either the rhamnose nucleotide syn thetic branch of I:he EPS biosynthesis or the pathway leading to glycolysis was observed for S. thermophilus LY03.