Carbohydrate metabolism in lactic streptococci: fate of galactose supplied in free or disaccharide form.

Phosphorylation of free galactose by lactic streptococci was mediated by an adenosine triphosphate (ATP)-dependent kinase. The phosphoenolpyruvate (PEP) phosphotransferase system (PTS) was involved to a limited extent in transport of the sugar. The conversion of free galactose to glucose also was demonstrated, and uridine diphosphogalactose-4-epimerase was demonstrated to account for this change. Galactose, supplied as lactose, was phosphorylated during transport by means of the PTS with PEP as the phosphate donor. Data also indicated that galactose derived from lactose was catabolized by the glycolytic pathway. Results showed the participation of ATP or PEP, or both, in the phosphorylation of five growth sugars for lactic streptococci, namely, galactose, glucose, lactose, maltose, and mannose. Free galactose was phosphorylated exclusively by ATP except when cells were grown on galactose; in this case, slight involvement of PEP in phosphorylation also was noted. Lactose phosphorylation was much more effective with PEP except when cells were grown on lactose, in which case ATP was equally effective. Glucose was phosphorylated to about the same degree by either ATP or PEP.