Osmotic pressure is the diffusion in which solvent and solute molecules pass through a semipermeable membrane till the concentration of the substance on either side of the membrane becomes equal. Osmotic pressure is defined as unbalanced pressure that gives rise to the phenomenon of diffusion and osmosis in a solution in which there are difference in concentration.
The solute concentration in a microbial cell is around 0.95% . if the cells are exposed to a solution having higher solute concentration, water will be drawn out of the cell. The cell becomes dehydrated , and the cytoplasmic membrane is drawn away from the cell wall with the cytoplasmic contents. This process is called plasmolysis.
If the microbial cells are suspended in a solution containing a very low concentration of salt, then water will flow from the solution in to the cell. Water continues to move inward until a balancing osmotic pressure is attained. This pressure may cause swelling , and result in bursting of the cell releasing the cellular constituents.the cell is then plasmoptyzed and the process is called plasmoptysis. The micro organisms usually do not exhibit striking changes because of their rigid cell wall and are less sensitive to osmotic effec as compared to the plant and animal cell. If destructive action has to be observed in the micro organisms a large increase in the osmotic pressure of the surrounding solution is required.
Micro organisms are generally inhibited by a high concentration of salt (10 to 15%) and sugar (50 to 70%). The technique of use of high osmotic pressure finds application in the preservation of foods like jam, jellies , condensed milk by use of concentrated sugar solution and preservation of meat, corned beef, fish , eetc are preserved by salting . here the mechanisms of microbial inhibition is plasmolysis. The cels are dehydrated and are unable to metabolise and grow. Micro organisms are not necessarily killed. Thus osmotic pressure is also microbiostatic adaptability of some micro organisms to varying osmotic pressure is quite amazing for eg some organisms are found in the oceans where salt concentration is quite high. Some micro organisms are found in the commercial pickling brines, where salt concentration is between 20 to 30% . such organisms that require high concentration of salt in media fro growing are called halophiles.
to be continued………