Ion transport and osmotic adjustment in E. coli in response to ionic and non-ionic osmotica

Bacteria respond to osmotic stress by a substantial increase in the intracellular osmolality, adjusting their cell turgor for altered growth conditions. Using E. 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 ~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 is 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 up-regulated 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. Two biological replicates per treatment with microarray analysis using the Affymetrix GeneChip E. coli Genome array. Treatments used included: Control - E. coli Frag1, grown to early stationary growth phase in a mineral salts medium with 0.1% glucose at 25 C Sucrose hyperosmotic treatment - 1.25 M sucrose added to control culture for 10 min. NaCl hyperosmotic treatment - 1.37 M NaCl added to control culture For microarray data comparisons the sucrose and NaCl hyperosmotic treatment data was compared to the no treatment control data separately. The sucrose to control and NaCL to control comparison data tables are linked below.