Agar
Say it: AH-gur
Agar is a jelly-forming polysaccharide extracted from red seaweeds, used above all to solidify culture media for growing microorganisms.
Also: agar-agar
Definition
What is agar?
Agar is a gelling material obtained from the cell walls of certain red algae, chiefly species of Gelidium and Gracilaria. Chemically it is a mixture of polysaccharides built from the sugar galactose: mainly agarose, which gives the gel its strength, and agaropectin, which carries sulfate groups. Agar does not dissolve in cold water but does in boiling water, and a solution of about 1.5 percent sets into a firm, clear gel as it cools to roughly 32 to 40 degrees Celsius. Once set, it does not melt again until heated to about 85 degrees.
These traits make it ideal for microbiology. Nutrients are mixed into hot agar, the medium is sterilized and poured into Petri dishes or tubes, and it stays solid at body temperature, where gelatin would melt. Very few microbes can digest agar, so the surface stays firm while colonies grow. Single cells spread over a plate grow into separate colonies, which is how pure cultures are isolated and counted. Added ingredients turn plain agar into selective or differential media, such as blood agar or MacConkey agar.
Agar entered bacteriology in Robert Koch's circle in the early 1880s, after Fanny Hesse suggested it from her kitchen. It is also a food thickener, and purified agarose forms the gels used to separate DNA by electrophoresis.
Word parts
Where does the word “agar” come from?
- agar
- seaweed jellyMalay agar-agar
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Terms that go with it
- Aseptic techniqueAseptic technique is the set of practices that keeps microbes from contaminating sterile materials, cultures or a patient's tissues during a procedure.
- BacteriumA bacterium is a single-celled prokaryote with no nucleus, found in almost every habitat on Earth, including in and on the human body.
- Gel electrophoresisGel electrophoresis is a lab technique that uses an electric field to pull DNA, RNA or proteins through a gel, separating them by size.
- CarbohydrateA carbohydrate is a sugar or a chain of sugars, such as starch or cellulose, built from carbon, hydrogen and oxygen and used for energy and structure.
- SterilizationIn microbiology, sterilization is any process that kills or removes all microbial life, including bacterial endospores and viruses, from an object.
- Gram stainThe Gram stain is a lab staining method that sorts bacteria into Gram-positive (purple) and Gram-negative (pink) groups based on their cell walls.
Sources
Checked against
Written for biofideX in plain words, after checking these references:
- OpenStax Microbiology, 9.6 Media Used for Bacterial Growth
- FAO, A Manual for the Processing of Agar from Gracilaria: Introduction
- NCBI MeSH: Agar
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