The Rate of Osmotic Shock Determines Bacterial Survival

Mechanosensitive (MS) channels allow cells to sense and respond to environmental changes. In bacteria, these channels are believed to protect against an osmotic shock. The physiological function of these channels has been primarily characterized by a standardized assay, where aliquots of batch cultured cells are rapidly pipetted into a hypotonic medium. Under this method, it has been inferred many types of MS channels (MscS homologs in E. coli) demonstrate questionable effectiveness against shock. We introduce a single-cell based assay which allows us to control how fast the osmolarity changes, over time scales ranging from a fraction of second to several minutes. We find that the protection provided by MS channels depends strongly on the rate of osmotic change, revealing that, under a slow enough osmotic drop, even "ineffective" MscS homologs can lead to survival rates comparable to those found in wild-type strains. Further, after the osmotic downshift, we observe multiple death phenotypes, which are inconsistent with the prevailing paradigm of how cells lyse. Both of these findings require a re-evaluation of our basic understanding of the physiology of MS channels