Osmosensing by bacteria: Signals and membrane-based sensors

Bacteria call survive dramatic osmotic shifts. Osmoregulatory responses mit igate the passive adjustments in cell structure and the growth inhibition t hat may ensue. The levels of certain cytoplasmic solutes rise and fall in r esponse to increases and decreases respectively, in extracellular osmolalit y. Certain organic compounds are favored over ions as osmoregulatory solute s, although K+ fluxes are intrinsic to the osmoregulatory response for at l east some organisms. Osmosensors must undergo transitions between "off" and "on" conformations in response to changes in extracellular water activity (direct osmosensing) or resulting charges ill cell structure (indirect osmo sensing). Those located in the cytoplasmic membranes and nucleoids of bacte ria are-positioned for indirect osmosensing. Cytoplasmic membrane-based osm osensors may detect changes in the periplasmic and/or cytoplasmic solvent b y experiencing changes in preferential interactions with particular solvent constituents, cosolvent-induced hydration changes, and/or macromolecular c rowding. Alternatively, the membrane may act as an antenna and osmosensors may detect changes in membrane structure. Cosolvents may modulate intrinsic biomembrane strain and/or topologically closed membrane systems may experi ence changes in mechanical strain in response to imposed osmotic shifts. Th e osmosensory mechanisms controlling membrane-based K+ transporters transcr iptional regulators, osmoprotectant transporters, and mechanosensitive chan nels intrinsic to the cytoplasmic membrane of Escherichia coli are under in tensive investigation. The osmoprotectant transporter ProP and channel MscL act as osmosensors after purification and reconstruction in proteoliposome s. Evidence that sensor kinase KdpD receives multiple sensory inputs is con sistent with the effects of K+ fluxes on nucleoid structure, cellular energ etics, cytoplasmic ionic strength, and ion composition as well as on cytopl asmic osmolality. Thus, osmoregulatory responses accommodate and exploit th e effects of individual cosolvents on cell structure and function as well a s the collective contribution of cosolvents to intracellular osmolality.