agar - Uses in microbiology, Uses in molecular biology, Uses in cooking, Uses in plant biology

A jelly-like compound produced from seaweed. It is used, after sterilization and the addition of suitable nutrients, for culture of fungi or bacteria for medicinal or research purposes.

Agar is an unbranched polysaccharide obtained from the cell walls of some species of red algae or seaweed. 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.

Uses in microbiology

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.

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 .

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.

One of the latest fad diets in Asia is the Kanten Diet.

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).

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.

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.

Other Uses of Agar

Agar is used as an impression material in dentistry.

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.