Selective advantages of various bacterial carbohydrate transport mechanisms

At least four strategies have been developed by bacteria for capturing carbohydrates. They are typified by the transport mechanisms for glycerol, glucose, lactose, and galactose in Escherichia coli. Glycerol enters the cell by facilitated diffusion catalyzed by specific membrane protein. Once inside the cell, the substrate is trapped by phosphorylation mediated by an adenosine triphosphate (ATP)-dependent kinase. glucose is phosphorylated in transit by a membrane-associated phosphoenolpyruvate phosphotransferase system (PTS). A specific component of this complex serves also for signal recognition to chemotaxis. Lactose is concentrated chemically unaltered by beta-galactoside permease driven by a proton motive force. Galactose is also pumped into the cell, but the process is energized by ATP or its equivalent. In addition, there is a periplasmic galactose-binding protein essential for both transport and chemotactic response. The relative functional merits of each kind of transport mechanism are discussed. Although many bacterial species possess both the concentrative mechanism and the PTS, some employ the former and others the latter for beta-galactoside utilization. The postulate that the PTS is more avid in scavenging while the concentrative permease system permits a broader range of substrate exploitation is supported by the growth responses of 12 bacterial strains to several beta-galactosides.