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Microorganisms don't feed on agar for the same reasons humans don't eat cardboard: the nutritional value simply isn't worth the effort. Scientists take advantage of this trait when they need to grow various microorganisms for research. Because agar has no nutritional value, researchers can precisely control the available nutrients in the agar plates used to culture organisms. This gives them a better understanding of the microorganisms they research, and it allows them to ensure that only certain kinds of organisms are permitted to grow.
Agar, also known as agar-agar, is a substance derived from certain types of red algae, more commonly called seaweed by laypersons. The Gelidium genus is particularly popular for commercial sources of agar. This gelatinous material has a number of interesting properties which have made it useful to humans; for example, agar is often used as a food additive to hold foods together without adding Calories. It is especially common in dairy desserts, where it is used as a thickener.
In scientific research, agar is used as a culture medium to grow a wide assortment of microorganisms. Researchers do this by making an agar plate, a dish filled with agar and a source of nutrition. Microorganisms are often very choosy about what they will eat, and the use of a controlled source of nutrition ensures that only certain types of microorganisms are allowed to grow. Unwanted interlopers have nothing to eat, since they can't feed on agar.
Researchers don't just use this substance because microorganisms can't feed on agar. It is also useful because it creates a convenient gel layer which allows them to clearly visualize the organisms they are growing. Additionally, it provides a convenient substrate for the microorganisms to grow on, creating small clusters known as colonies which can be easily picked off the plate for further research and other uses.
Growing microorganisms is hard work. Often, it takes several tries to find a source of nutrition which satisfies an organism, and the formula may need to be continually refined, with additional adjustments being made to the light, temperature, and humidity in the growing environment. Several scientific companies sell basic agar plate kits with common culture mediums, but advanced labs often have to custom-blend their agar plates to satisfy the needs of their research subjects.
Because microorganisms don't feed on agar, agar plates are also inherently sterile until nutrients are added. Maintaining a sterile environment is critical to many branches of scientific research, since scientists want to make sure that they know exactly what they are studying. Since microorganisms don't feed on agar, scientists know that a major potential variable is removed from their research when they use properly prepared agar plates.