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What Is the Purpose of Agar in Nutrient Agar?

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Microbiologists need to grow bacteria in the laboratory for a variety of different reasons in order to perform research experiments. In order to do this in the most effective way, it is important to use a growth medium that is as close as possible to the natural environment. Nutrient broth with agar is called nutrient agar. The overall purpose of the agar is to customize the media for the specific bacteria.

Nutrient Broth

Nutrient broth is made up of water, beef extract, and peptone. Peptone is a nitrogen mixture of plant and animal sources. Agar is then added to make the mixture into a solid and is called nutrient agar.


Agar is a polysaccharide that comes from marine red algae. Polysaccharides are repeating chains of monosaccharides linked together by glycosidic bonds. The agar remains a liquid colloid until it is cooled to below 36 degrees Celsius. A colloid is simply a mixture containing particles dispersed throughout and too small to be seen by the unaided eye.

Purpose of the Agar

Agar does not become a solid until it reaches a certain temperature, which is useful so that blood can mix with the culture media. This procedure specifically allows one to determine hemolytic reactions. A solid agar can also be useful for growing bacteria, isolating pure cultures, or measuring bacterial growth.

Agar for Testing Motility

If you are trying to test something that requires bacteria to have mobility, then a more semisolid medium is needed. A lower concentration of agar can be used in order to produce this result.

Other Considerations

Once the agar becomes solid, it will remain solid at room temperature. At a temperature of 85 degrees Celsius or above, it will melt. Thermophilic bacteria can be grown effectively on an agar because of this property.

  • "Alcamo's Fundamentals of Microbiology"; Jeffrey C. Pommerville; 2004
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