Multiphase Recovery of Escherichia Coli to Hyperosmotic Shock

Abstract

All living cells employ an array of different mechanisms to help them survive changes in extracellular osmotic pressure. The difference in the concentration of chemicals in a bacterium's cytoplasm and the external environment generates an osmotic pressure that inflates the cell. It is thought that the bacterium Escherichia coli uses a number of interconnected systems to adapt to changes in external pressure. This allows these cells to maintain turgor and live in surroundings that range more than two-hundred-fold in external osmolality. Here, we use fluorescence imaging to make the first measurements of cell volume changes over time during hyperosmotick shock and subsequent adaptation on a single cell level in vivo with a time resolution on the order of seconds. We directly observe two previously unseen phases of the cytoplasmic water efflux upon hyperosmotic shock and identify the mechanisms behind them. Furthermore, we monitor cell volume changes during the post-shock recovery and observe two different types of response that depend on the shock level, as well as two different recovery time scales. The initial phase of recovery is fast, on the order of 20 min, and proceeds even in the absence of the two potassium transporters TrkA(G/H) and KdpFABC. A protein-synthesis controlled mechanism then causes the cell to switch to a second, slower recovery phase that lasts on the order of several hours. Interestingly, the occurrence of this secondary phase requires the presence of both TrkA(G/H) and KdpFABC, suggesting that it is triggered by coordinated import of potassium using these two transporters.