Agar and agarose

Agar and agarose are two forms of solid growth media that are used for the culture of microorganisms , particularly bacteria . Both agar and agarose act to solidify the nutrients that would otherwise remain in solution. Both agar and agarose are able to liquefy when heated sufficiently, and both return to a gel state upon cooling.

Solid media is prepared by heating up the agar and nutrient components so that a solution results. The solution is then sterilized, typically in steam-heat apparatus known as an autoclave. The sterile medium is then poured into one half of sterile Petri plates and the lid is placed over the still hot solution. As the solution cools, the agar or agarose becomes gel-like, rendering the medium in a semi-solid. When bacteria contact the surface of the medium, they are able to extract the nutrients from the medium and grow as colonies.

The use of agar and agarose solid media allows for the isolation of bacteria by a streak plate technique. A similar discrimination of one bacterial species from another is not possible in liquid growth media. Furthermore, some solid growth media allows reactions to develop that cannot develop in liquid media. The best-known example is blood agar , where the total and partial destruction of the constituent red blood cells can be detected by their characteristic hemolytic reactions.

Agar is an uncharged network of strands of a compound called gelactose. This compound is in fact made up of two polysaccharides called agarose and agaropectin. Gelactose is extracted from a type of seaweed known as Gelidium comeum. The seaweed was named for the French botanist who first noted the gelatinous material that could be extracted from the kelp . Another seaweed called Gracilaria verrucosa can also be a source of agar.

Agarose is obtained by purification of the agar. The agarose component of agar is composed of repeating molecules of galactopyranose. The side groups that protrude from the galactopyranose are arranged such that two adjacent chains can associate to form a helix. The chains wrap together so tightly that water can be trapped inside the helix. As more and more helices are formed and become cross-linked, a three-dimensional network of water-containing helices is created. The entire structure has no net charge.

The history of agar and agarose extends back centuries and the utility of the compounds closely follow the emergence and development of the discipline of microbiology. The gel-like properties of agar are purported to have been first observed by a Chinese Emperor in the mid-sixteenth century. Soon thereafter, a flourishing agar manufacturing industry was established in Japan. The Japanese dominance of the trade in agar only ended with World War II. Following World War II, the manufacture of agar spread to other countries around the globe. For example, in the United States, the copious seaweed beds found along the Southern California coast has made the San Diego area a hotbed of agar manufacture. Today, the manufacture and sale of agar is lucrative and has spawned a competitive industry.

The roots of agar as an adjunct to microbiological studies dates back to the late nineteenth century. In 1882, the renowned microbiologist Robert Koch reported on the use of agar as a means for growing microorganisms. Since this discovery, the use of agar has become one of the bedrock techniques in microbiology. There are now hundreds of different formulations of agar-based growth media. Some are nonspecific, with a spectrum of components present. Other media are defined, with precise amounts of a few set materials included. Likewise the use of agarose has proved tremendously useful in electrophoretic techniques. By manipulation of the formulation conditions, the agarose matrix can have pores, or tunnels through the agarose strands, which can be of different size. Thus the agarose can act as a sieve, to separate molecules on the basis of the size. The uncharged nature of agarose allows a current to be passed through it, which can drive the movement of samples such as pieces of deoxyribonucleic acid (DNA ) from one end of an agarose slab to the other. The speed of the molecule movement, is also related to molecular size (largest molecules moving the least).

In the non-microbiological world, agar and agarose have also found a use as stabilizers in ice cream, instant cream whips, and dessert gelatins.

See also Bacterial growth and division; Laboratory techniques in microbiology