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When nutrient media are prepared, the various ingredients are dissolved in distilled water, not tap water. Distilled water is used in order to minimize the presence of excess inorganic salts or extraneous organic compounds that may be in a tap water.

Since tap water may vary in its mineral and dissolved solids composition from day to day, more consistent preparations can be obtained utilizing distilled water. Generally speaking, the osmotic pressure (osmolarity) of a suitably prepared medium is comparable or at least does not vary excessively from that found in the microorganisms, a condition referred to as being isotonic.

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Within reasonable limits, changes is osmotic pressure, due to the utilization of a medium’s components or because of a partial declaration of the medium, are not usually sufficient to prevent the growth of the organism.

When cells are taken from an isotonic situation and placed in distilled water at lower osmotic pressure (a hypotonic condition), water will enter the cells.

This is caused by the greater concentration of dissolved substances within the cell in comparison to the distilled water. Since the dissolved solids cannot distribute themselves because of the microbial membrane, the water is attracted and enters the cells.

In most cases the bacterial cell wall is rigid enough to withstand the increased pressure caused by the incoming water molecules. However, some cells will burst, a condition called plasmoptysis.

Plasmoptysis occurs more readily with animal cells than with plant or microbial cells, because of the absence of a rigid cell wall. Probably the best example of this reaction is the production of red blood cells stroma, or ghosts, caused by introducing whole blood into distilled water.

The cell membranes are stretched sufficiently to release most of their cytoplasmic components. Apparently little holes are temporarily created during this reaction which subsequently close or heal.

When cells are placed into a solution of greater osmolarity or hypertonicity, water molecules escape from them, thus creating a shrinkage known as plasmolysis.

In this phenomenon, the cytoplasm becomes concentrated and will generally pull away from the cell wall of Gram-negative bacteria but not necessarily of Gram-positive bacteria. With erythrocytes, cells without walls, entire units shrink. Four factors are to be taken into account.

1. Media providing optimum growth.

2. Temperature.

3. Atmosphere.

4. Cultural characteristic, e.g., size, shape and pigmentation of colonies. Media can be (a) basic (b) enrichment (c) selective and (d) indicator media.