Lactococcus cremoris is a lactic acid bacterium that is used in dairy applications such as a strarter for cheese production, where lactate acidifies the milk as the key product produced from sugars. The central metabolism of L. cremoris has been studied intensively as a model system, in particular because of its interesting metabolic switching behaviour: Under sugar excess - or more precisely, high glycolytic flux conditions - lactate is produced by homolactic fermentation. However, when glucose is limited (e.g. in a glucose limited chemostat), or in the presence of a “slow” sugar, mixed acid fermentation occurs with formate, acetate and ethanol as products.

Since enzyme concentrations hardly vary in the chemostat when the switch occurs, metabolic regulation is believed to underlie the shift. However, a satisfying and unifying explanation of the switch, and how glycolysis is regulated in this bacterium remains elusive, despite a number of kinetic models in the literature. We revisited these models and designed a kinetic model of glycolysis with a focus on growth-associated ATP supply and demand, phosphate homeostasis and enzyme kinetics that captures the most important regulatory mechanisms known from literature. Our model reproduces the metabolic shift and is able to reproduce and explain many different experimental results regarding to the control and regulation of the glycolytic flux and its branches.