Organic vs inorganic

In order to survive hyperosmotic stress bacteria should adjust their cell turgor to altered conditions by increasing the intracellular osmolality. The classical view is that bacterial osmotic adjustment is achieved via accumulation of so-called “compatible solutes”— some organic osmolytes that can be accumulated in the cytosol at high concentrations without interfering with cell metabolism. In our recently published paper,11 we have shown that in the absence of osmolytes in the environment uptake of inorganic ions (and, specifically, K+) is central to osmotic adjustment in E. coli under hyperosmotic stress conditions. Here we show that optimal E. coli growth, observed at 2% NaCl, corresponds to an osmotic balance between external and internal osmolality within bacterial cells. This is achieved by the regulation of net K+ fluxes across the bacterial membrane. We suggest that the role of compatible solutes in osmotic adjustment in bacteria is indirect and confined to the fine tuning of a number of ion channels and transporters in order to achieve osmotic balance.