In bacterial growth, water availability is controlled by:
Water availability parameter aw water activity influences growth because intracellular reactions require aqueous medium, turgor pressure drives expansion. aw equals relative humidity 100 percent in equilibrium. Bacteria regulate response to aw change mainly via osmotic pressure sensing osmosensors EnvZ. Hyperosmotic shock water exits cytoplasm plasmolysis outer membrane separates, cytoplasm shrivels, growth arrested. Compatible solutes accumulation potassium glutamate, proline, ectoine, trehalose synthesised or imported via ProP, BetT transporters balances osmolarity without denaturing protein
Ref: Brock Biology of Microorganisms, 16th ed., Chapter 5: Water activity and osmotic pressure.