Transporters and their roles in LAB cell physiology.

For most metabolic pathways, the uptake of the substrate into the cell represents the first step. This transport reaction can exert a large control on the flux through the pathway, in particular when the substrate concentration becomes limiting. Besides serving a role in the uptake of nutrients and the excretion of metabolic (end)products or drugs, transport systems can have one or more other functions in the physiology of the cell. Two of these functions, control of carbohydrate utilization and regulation of cell volume, have been well established in lactic acid bacteria (LAB). The first example concerns the phosphoenolpyruvate-dependent phosphotransferase system (PTS), which serves a role in the transport of sugars into the cell but also regulates the activity of metabolic pathways, either through regulation of transcription and/or (in)activation of transporters and key enzymes already present. The regulation by the PTS results in a hierarchy in the utilization of sugars and/or adjustment of the first step(s) of a metabolic pathway to the metabolic capacity of the cell and the availability of a particular substrate. The second example relates to the activation of transporters (and mechanosensitive channels), which represents the first mechanism of defence against osmotic stress. The activation by osmotic-upshift of the ATP-binding Cassette (ABC) transporter OpuA from Lactococcus lactis is compared with the activation by osmotic-downshift of mechanosensitive channels. The mechanosensitive channels have been best studied in organisms other than LAB, but the presence of similar systems in LAB, and their conservation of structure, suggest that the postulated functions and mechanisms generally hold.