Agar is a seaweed polysaccharide extract that is used as a solidifying agent in many types of food and desserts. It has also been used for over 100 years as a solid substrate for the growth of bacteria in Petri plates when combined with other essential nutrients for growth. It is widely used because it is inexpensive and easy to use.
Facts
Agar is substance derived from seaweed that can be added to food and industrial products as a gelling agent. It is a polysaccharide, or a polymer of the carbohydrate galactose, and is a primary component of the cell walls of seaweed. It is very popular in the cuisine of Asian countries and is used in ice cream, desserts and soups. Unlike gelatin, agar is not derived from animal sources and is a good substitute for vegetarians and vegans.
Use in Microbiology
In addition to its uses in food, agar has a very important role in microbiology, the study of microscopic organisms, including bacteria. Agar is used in laboratories as a solid surface for the growth of bacteria on Petri plates or in test tubes. Typically, agar powder is dissolved into water along with the required nutrients for growth of the desired microorganism. Other compounds such as antibiotics may be added to the agar mixture to test susceptibility of a particular strain of bacteria.
Metabolism by Microbes
Agar is an effective agent for providing a structure for the growth of bacteria because most strains are unable to metabolize it. This means that the gelling capabilities of agar are not usually affected by the growth of bacteria on its surface. However, a report as early as the one from 1941 in the "Journal of Bacteriology" notes there are many species of bacteria that are able to metabolize agar, making these types unsuitable for growth on agar plates.
Alternatives to Agar
There are alternative gelling agents to agar that may be used if it is not available or if the species of bacteria of interest can metabolize it. Gelatin may also be used as a solidifying agent, but many bacteria can metabolize it, which disturbs the structure as the bacteria grow over time. Another agent called gellan gum can be used and is more heat-tolerant than agar. Gellan is described in 1989 in the journal "Pure and Applied Chemistry." Another advantage of gellan gum is that it is a much more effective gelling agent, so much less is required than agar. In general, agar is the cheapest and most common solid substrate for growth of bacteria in a laboratory setting.
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