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Bacteria respond to osmotic stress by a substantial increase in the intracellular osmolality, adjusting their cell turgor for altered growth conditions. Using Escherichia coli as a model organism we demonstrate here that bacterial responses to hyperosmotic stress specifically depend on the nature of osmoticum used. We show that increasing acute hyperosmotic NaCl stress above approximately 1.0 Os kg(-1) causes a dose-dependent K(+) leak from the cell, resulting in a substantial decrease in cytosolic K(+) content and a concurrent accumulation of Na(+) in the cell. At the same time, isotonic sucrose or mannitol treatment (non-ionic osmotica) results in a gradual increase of the net K(+) uptake. Ion flux data are consistent with growth experiments showing that bacterial growth is impaired by NaCl at the concentration resulting in a switch from net K(+) uptake to efflux. Microarray experiments reveal that about 40% of upregulated genes shared no similarity in their responses to NaCl and sucrose treatment, further suggesting specificity of osmotic adjustment in E. coli to ionic and non-ionic osmotica. The observed differences are explained by the specificity of the stress-induced changes in the membrane potential of bacterial cells highlighting the importance of voltage-gated K(+) transporters for bacterial adaptation to hyperosmotic stress.

A group of pigmented, psychrophilic, strictly aerobic chemoheterotrophs isolated from sea-ice cores collected from coastal areas of eastern Antarctica was found to represent a novel 165 rRNA lineage within the gamma subclass of the Profeobacferia, adjacent to the genus Alteromonas. The isolates are motile, Gram-negative, rod-shaped cells, which are psychrophilic and slightly halophilic, and possess an absolute requirement for seawater. Differences in phenotypic characteristics and DNA-DNA hybridization indicated the isolates formed two distinct taxa which have DNA G+C contents of 4 4 4 6 mol% and 40 mol%, respectively. Whole-cell fatty acid profiles of the isolates were however very similar and included 16:lco7c, 18:lcv7c, 16:O and 17:lco8cas the major fatty acid components. Overall, sufficient differences exist to distinguish the sea-ice strains from currently recognized bacterial genera. It is proposed the sea-ice strains represent a new genus, Glaciecola, which contains two species, Glaciecola punicea gen. nov., sp. nov. (ACAM 61 IT) and Glaciecola pallidula gen. nov., sp. nov. (ACAM 615T).

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