Lactose metabolism in lactic acid bacteria

Since cheese is a fermented dairy product, lactose metabolism by lactic acid bacteria has been thoroughly studied and is now well understood. Lactococcus lactis transports lactose into the cell using the phosphoenolpyruvate phosphotransferase system which phosphorylates lactose to lactose-6-phosphate. Lactococci possess a phospho-beta-galactosidase which hydrolyses lactose-6-phosphate to glucose and galactose-6-phosphate. The latter is converted, via the tagatose pathway, to dihydroxyacetone-phosphate which is isomerised to glyceraldehyde-3-phosphate and thus into the glycolytic pathway. The glucose moiety of lactose is metabolised to lactic acid by glycolysis. Lactococci produce 4 mol L-lactate per mol lactose.

Most other lactic acid bacteria transport lactose into the cell as lactose using a permease. These organisms hydrolyse lactose to glucose and galactose using a beta-galactosidase; galactose is converted to glucose-6-phosphate by the Leloir pathway and thus into the glycolytic pathway. Streptococcus thermophilus metabolises 1 mol lactose to 2 mol L-lactate since they metabolise only the glucose moiety of the disaccharide. Certain species of lactobacilli produce D- or DL-lactate, depending on the type of lactate dehydrogenase they possess. Leuconostoc sp. use a different sequence of biochemical events for lactose metabolism; the end products of phosphoketolase pathway are quite different (1 mol lactose is converted to 2 mol D-lactate, 1 mol ethanol and 2 mol CO2) to those of glycolysis.

Further reading:

Fox, P.F., T.P. Guinee, T.M. Cogan and P.L.H. McSweeney (2000). Fundamentals of Cheese Science. Aspen Publishers, Gaithersburg, MD. 587 pp