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agar(ā′gär′, ä′gär′) also
A gelatinous, sulfated polysaccharide extracted primarily from Gelidium cartilagineum, Gracilaria confervoides, and related species of red algae (seaweed); it melts at ±100ºC and solidifies at ±40ºC.
Herbal medicine Agar has been used as a bulk laxative, as it is highly hydrophilic.
Microbiology Agar is the most commonly used support medium for bacterial and fungal culture, as nutrients, antibiotics, salts and various growth enhancers and inhibitors are easily incorporated into the media.
Agar is also used as an emulsifier in foods; it cannot be digested by humans.
Australian Group on Antimicrobial Resistance Study. An ongoing surveillance of antimicrobial resistance in Australian teaching hospitals,which began in 1986.
agarA seaweed extract, sometimes called agar-agar, much used in bacteriological laboratories because it forms a convenient gel for the suspension of nutrient culture material, such as blood or broth, on which micro-organisms can be grown in an incubator.
agara complex POLYSACCHARIDE obtained from marine algae, which is widely used (in gel form) as a solidifying agent. Agar has two main components, agarose and agaropectin. Agar is used in various kinds of microbiological MEDIUM, and refined forms of agar or agarose are used in techniques such as ELECTROPHORESIS and gel filtration. In industry it is used as a gelling agent in foods such as jellies, soups and ice cream.
Agar has certain properties that make it particularly valuable in MICROBIOLOGY:
- it is translucent or transparent and is degraded by only a few MICROORGANISMS.
- it melts at about the boiling point of water, but remains liquid until the temperature has dropped to about 40–45 °C, when gelling occurs. Thus it can be poured over or mixed with a bacterial INOCULUM at about 50 °C, without injuring the bacteria. Once it has solidified it can be incubated at temperatures up to about 65 °C, perhaps higher, without liquifying. This is particularly useful where THERMOPHILIC microorganisms are being grown.
Agar medium is prepared by adding agar, often before autoclaving (see AUTOCLAVE), to the nutrients etc. of the medium. Agar medium is generally contained in a PETRI DISH (plate) or test tube. The test tubes containing agar are called ‘slants’ or ‘slopes’ when they allow the medium to set at an angle. When the agar solidifies in a vertical tube it is called a ‘deep’. In a Petri dish the medium solidifies as a layer over the base of the dish.