This chapter focuses on potassium (K+) transport in bacteria. K+ is a “compatible” cation, best tolerated of monovalent cations when present in the cell at high concentration. This tolerance allows K+ to make a major contribution to regulating cell osmolality as supported by the correlation between internal K+ and medium osmolality in several species. Bacteria have multiple transport systems for K +. Multiplicity of systems, each with characteristic properties, allows cells to adapt to a wider range of environments. Systems for influx are separate from systems for efflux. A single system does not appear to mediate movements in both directions. Stimulation of K+ transport is an early, or the earliest, event in many bacteria when confronted with a change in medium osmolality. A scheme of osmotic adaptation for E. coli and the cyanobacteria is shown diagrammatically in the chapter. Feedback-regulated transport readily accounts for the way a cell maintains a fixed K+ concentration during growth. The increase in cell volume during growth dilutes cell solutes, thereby reducing turgor pressure.
