Metabolism associated with raised metabolic flux to sugar nucleotide precursors of exopolysaccharides in Lactobacillus delbrueckii subsp. bulgaricus

Abstract

Exopolysaccharide (EPS) metabolism was studied in a galactose-negative strain of Lactobacillus delbrueckii subsp. bulgaricus, using two different approaches. Firstly, using both the parent strain and a chemically induced mutant with higher yield and specific productivity of EPS than the parent, comparative information was obtained relating to enzyme activities and metabolite levels associated with EPS formation when grown on lactose. Under continuous culture conditions (D = 0.10 h(-1)), the higher metabolic flux towards EPS formation in the mutant strain relative to the parent appeared to be mediated by raised levels of UDP-glucose pyrophosphorylase (UGP). Marginally raised UDP-galactose 4-epimerase (UGE) activity in the mutant strain suggested that this enzyme could also play a role in EPS overproduction. The second approach involved investigating the effect of growth rate on sugar nucleotide metabolism in the parent, as it is known that EPS production is growth-associated in this strain. UGE activity in the parent strain appeared to increase when the growth rate was elevated from 0.05 to 0.10 h(-1), and further to 0.35 h(-1), conditions that can be associated with higher levels of metabolic flux to EPS formation. Concurrent with these increments, intracellular ATP levels in the cell were raised. In both investigations glucose-6-phosphate accumulated pointing to a constriction at this branch-point, and a limitation in the flow of carbon towards fructose-6-phosphate or glucose-1-phosphate. The changes in metabolism associated with enhanced flux to EPS provide guidance as to how the yield of Lactobacillus delbrueckii subsp. bulgaricus EPS can be improved.