pH - most bacteria must be grown in media with high water...

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pH Most bacteria grow best at a pH of 6.5 - 7.5 (neutral or near neutral). Most bacteria do not grow  at all below a pH of about 4 but a few acidophiles do tolerate acidity.  Molds and yeasts prefer a pH of 5 - 6, but tend to grow at least some over a wide range of pH.  Acid foods such as pickles and sauerkraut usually do not undergo bacterial spoilage. Alkalinity  could also be used to preserve food, but high pH tends to make foods bitter and slimy, so this  method of preservation is not desirable. As bacterial cultures grow in a closed container, they frequently produce acids as waste products,  so microbiological lab media usually contain buffers to prevent drastic shifts in pH.    OSMOTIC PRESSURE If bacteria are surrounded by a solution with a high osmotic pressure (high solute concentration),  water tends to leave the bacterial cell (plasmolysis). This can slow growth or even kill the cell, so 
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Unformatted text preview: most bacteria must be grown in media with high water and low solute content. High levels of salt or sugar can be used for food preservation. There are some exceptions to this: Extreme halophiles—tolerate very high salt concentrations Obligate halophiles—actually require an extremely high salt concentration for growth, possibly up to 30% salt Facultative halophiles—can grow at salt concentrations up to 2%, but do not require them. A few of these can even tolerate 15% salt. Although bacteria frequently find themselves in surroundings with a lower osmotic pressure than that inside the cell, the cell wall usually prevents excessive water from entering the cell. If osmotic pressure of a solution is extremely low, such as pure distilled water, some microbes might be damaged....
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