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 proteins. Mechanosensitive channels MscL MscS open to release solutes during hypoosmotic downshock preventing lysis. Halophiles require high sodium for protein solvation acidic proteome. Temperature indirectly affects aw through evaporation, oxygen concentration affects respiration not hydration, barometric pressure affects gas solubility. Thus osmotic pressure is primary determinant controlling water availability defining limits in dried foods jerky aw 0.8, salted cod, brines, desert soils. Food industry reduces aw by adding salt sugar to inhibit spoilage microbes and extend shelf life without refrigeration.
Ref: Brock Biology of Microorganisms, 16th ed., Chapter 5: Water activity and osmotic pressure.