Agar

Overview
Agar is a gelatinous substance chiefly used as a culture medium for microbiological work. It is an unbranched polysaccharide obtained from the cell walls of some species of red algae or seaweed. It can be used 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. 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.

Uses in microbiology
Nutrient agar is used throughout the world as a medium for the growth of bacteria and fungi. 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 some microbes prefer certain environmental conditions over others. For example, blood agar, which is generally combined with sheep blood, can be used to detect the presence of haemorrhagic micro-organisms such as Escherichia coli O157:H7. 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.

Molecular biology
Agar is a heterogeneous mixture of two classes of polysaccharide: agaropectin and agarose. 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 in agarose gel electrophoresis or for column-based gel filtration chromatography.

Culinary
Agar-Agar is the sea's natural gelatin. White and semi-translucent, it is sold in packages as washed and dried strips or in powdered form. It can be used to make jellies, puddings and custards. For making jelly, it is boiled in water until the solids dissolve. One then adds sweetener, flavouring, colouring, fruit or vegetables, and pours the liquid into molds to be served as desserts and vegetable aspics, or incorporated with other desserts, such as a jelly layer on a cake.

Agar-agar is approximately 80% fiber, so it can serve as a great intestinal regulator. Its bulk quality is behind one of the latest fad diets in Asia, 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.

In Indian cuisine, agar agar is known as "China Grass" and is used for making desserts.

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.

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 cycle 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 may 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.

Other
Agar is used as an impression material in dentistry. It is also used to make salt bridges for use in electrochemistry.

Hysteresis
Hysteresis describes the phenomenon of the differing liquid-solid state transition temperatures that agar exhibits. Agar melts at 85 °C (358 K, 185 °F) and solidifies from 32-40 °C. (305 - 313 K, 90-104 °F)