agar

For other uses, see Agar (disambiguation).

Agar is an unbranched polysaccharide obtained from the cell walls of some species of red algae or seaweed. The word agar comes from the Malay word agar-agar (meaning jelly). It is also known as kanten or agal-agal (Ceylon agar). Chemically, agar is a polymer made up of subunits of the sugar galactose. Agar polysaccharides serve as the primary structural support for the algae's cell walls. Dissolved in hot water and cooled, agar becomes gelatinous. Its chief use is as a culture medium for microbiological work. Other uses are as a laxative, a vegetarian gelatin substitute, a thickener for soups, in jellies, ice cream and Japanese desserts such as anmitsu, as a clarifying agent in brewing, and for paper sizing fabrics.

Uses in microbiology

Main article: Agar plate

Nutrient agar is used throughout the world as a medium for the growth of bacteria and fungi, but not viruses (however, viruses are often grown in bacteria that are growing on agar). Though less than 1% of all existing bacteria can be grown successfully, the basic agar formula can be used to grow most of the microbes whose needs are known. More specific nutrient agars are available, because microbes can be picky. For example, blood agar, which is generally combined with horse blood, can be used to detect the presence of haemorrhagic micro-organisms such as E.coli O:157 H:7. The bacteria digest the blood, turning the plate clear.

Selective media

Selective media is agar specially treated to apply a selective pressure to organisms growing on it -- for example, to select for salt-tolerant, gram-positive, or gram-negative bacteria. To select for only gram negative organisms you would use MacConkey agar, which would also in turn tell you if the gram negative organism is a lactose fermenter or not indicated by red colonies instead of translucent (non- lactose fermenter).

Differential media

Differential media includes an indicator that causes visible, easily detectable changes in the appearance of the agar gel or bacterial colonies in a specific group of bacteria. For example, EMB (Eosin Methylene Blue) agar causes E. coli colonies to have a metallic green sheen, and MSA (Mannitol Salt Agar) turns yellow in the presence of mannitol fermenting bacteria.

Uses in molecular biology

Agar is a heterogeneous mixture of two classes of polysaccharide: agaropectin and agarose ^[1]. Although both polysaccharide classes share the same galactose-based backbone, agaropectin is heavily modified with acidic side-groups, such as sulfate and pyruvate. The neutral charge and lower degree of chemical complexity of agarose make it less likely to interact with biomolecules, such as proteins. Gels made from purified agarose have a relatively large pore size, making them useful for size-separation of large molecules, such as proteins or protein complexes >200 kilodaltons, or DNA fragments >100 basepairs. Agarose can be used for electrophoretic separation or for column-based gel filtration chromatography.

Uses in cooking

Agar is typically sold as packaged strips of washed and dried seaweed, or in powdered form. Raw agar is white and semi-translucent. For making jelly, it is boiled in water at a concentration of about 0.7-1% w/v (e.g. a 7 gram packet of powder into 1 litre of water would be 0.7%) until the solids dissolve, after which sweeteners, flavouring, colouring, and pieces of fruit may be added. The agar-agar may then be poured into molds or incorporated into other desserts, such as a jelly layer on a cake.

One of the latest fad diets in Asia is the Kanten Diet. Once ingested, kanten triples in size and absorbs water. This results in the consumer feeling more full. Recently this diet has received some press coverage in the United States as well. The diet has shown promise in obesity studies, but agar/kanten has virtually no nutritional value. It is approximately 80% fiber, so part of the diet's effectiveness may be a result of it working as a laxative. There are also some (claimed) effects as to the benefits of agar-agar in controlling diabetes.

Uses in plant biology

Research grade agar is used extensively in plant biology as it is supplemented with a nutrient and vitamin mixture that allows for seedling germination in petri dishes under sterile conditions (given that the seeds are sterilized as well). Nutrient and vitamin supplementation for Arabidopsis thaliana is standard across most experimental conditions. Murashige & Skoog (MS) nutrient mix and Gamborg's B5 vitamin mix are generally used. A 1.0% agar/0.44% MS+vitamin dH20 solution is suitable for growth media between normal growth temps.

It is important to note that the solidification of the agar within any growth media (GM) is pH-dependent, with an optimal range between 5.4-5.7. Usually, the application of KOH is needed to increase the pH to this range. A general guideline is about 600 µl 0.1M KOH per 250 ml GM. This entire mixture can be sterilised using the liquid cyle of an autoclave.

This medium nicely lends itself to the application of specific concentrations of phytohormones etc. to induce specific growth patterns in that you can easily prepare a solution containing the desired amount of hormone, add it to the known volume of GM and autoclave to both sterilize and evaporate off any solvent you mave have used to dissolve the often polar hormones in. This hormone/GM solution can be spread across the surface of petri dishes sown with germinated and/or etiolated seedlings.

Hysteresis

Hysteresis describes the phenomenon of the differing liquid-solid state transition temperatures that agar exhibits. Agar melts at 85 °C and solidifies from 32-40 °C.

See also

• Agar plate
• Rhodophyta
• Asepsis
• Microbiology

External links

• General agar information
• Adzuki Bean Agar-Agar Recipe
• Chemical structure of agarose

References

1. ^ FAO agar manual