Microbiology and Immunology

Agar and agarose WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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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 dis-

crimination of one bacterial species from another is not possi-

ble in liquid growth media. Furthermore, some solid growth

media allows reactions to develop that cannot develop in liq-

uid 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 verrucosacan also

be a source of agar.

Agarose is obtained by purification of the agar. The

agarose component of agar is composed of repeating mole-

cules 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 man-

ufacture and sale of agar is lucrative and has spawned a com-

petitive industry.

The roots of agar as an adjunct to microbiological stud-

ies dates back to the late nineteenth century. In 1882, the

renowned microbiologist Robert Kochreported on the use of

agar as a means for growing microorganisms. Since this dis-

covery, the use of agar has become one of the bedrock tech-

niques in microbiology. There are now hundreds of different

formulations of agar-based growth media. Some are nonspe-

Aerobic fungus growing on agar.

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