Galactose fermentation by Streptococcus lactis and Streptococcus cremoris: pathways, products, and regulation

Area of Science:

• Microbiology
• Biochemistry
• Metabolic Engineering

Background:

• Lactic streptococci are key in dairy fermentation.
• Galactose metabolism in these bacteria can lead to diverse end products.
• Understanding these pathways is crucial for optimizing fermentation processes.

Purpose of the Study:

• To investigate the fermentation products of galactose by various lactic streptococci strains.
• To elucidate the metabolic mechanisms underlying differential galactose utilization.
• To identify factors influencing the diversion of lactate to other fermentation products.

Main Methods:

• Cultivation of Streptococcus lactis and Streptococcus cremoris strains with galactose.
• Analysis of fermentation end products (lactate, formate, acetate, ethanol) using chromatography.
• Enzyme activity assays and measurement of intracellular metabolite levels (fructose 1,6-diphosphate, triose phosphates).

Main Results:

• Most lactic streptococci fermented galactose to lactate, formate, acetate, and ethanol.
• Streptococcus lactis ML8 exclusively produced lactate from galactose.
• Reduced fructose 1,6-diphosphate and elevated triose phosphates in most strains diverted metabolism from lactate.
• Streptococcus cremoris strains showed reduced growth at low galactose concentrations due to specific uptake and metabolism pathways.
• Streptococcus lactis strains exhibited varied galactose uptake and metabolism depending on external galactose concentration.

Conclusions:

• Intracellular metabolite levels, particularly fructose 1,6-diphosphate and triose phosphates, regulate galactose fermentation pathways in lactic streptococci.
• Specific strains like S. lactis ML8 maintain metabolic flux towards lactate production.
• Galactose uptake and metabolism mechanisms differ between S. cremoris and S. lactis, impacting growth and fermentation.