23 Dec

Water Availability

Water availability, defined by water activity, can have a large effect on the growth of bacteria.  The value of water activity is between 0 and 1.0.  The closer the value is to 1.0, the more water is available to a cell for metabolic purposes.  Water activity and availability decrease with increases in concentration of solutes like salts.  In a hypotonic solution the solute concentration is lower outside of the cell than it is on the inside of the cell.  The cell wall of bacteria protects the cell against the osmotic pressure.  When the solute concentration is greater on the outside of the cell relative to the inside, the solution is hypertonic.  Water will leave the cell during the process of plasmolysis.  Bacteria can be grouped based on their ability to cope with low water activity and high osmotic pressure.  Halophiles require high concentrations of sodium chloride to grow.  Some of the bacteria in this group require 15-30% sodium chloride to maintain the integrity of their cell walls.  Bacteria that belong to the group osmophiles can grow in environments where sugar concentrations are excessive.