[EBOOK] Nutrient Agar for microbiology

Nutrient agar is used as a general purpose medium for the growth of a wide variety of non-fastidious microorganisms. It consists of peptone, beef extract and agar. This relatively simple formulation provides the nutrients necessary for the replication of a large number of non-fastidious microorganisms.

Nutrient Agar/broth is used for the cultivation and maintenance of non-fastidious organisms as well as enumeration of organisms in water, sewage, dairy products, feces and other materials.

With its distinctive smell, one can easily distinguish agar from the other materials commonly found in a laboratory. Chemically, agar is a polymer made up of subunits of the sugar galactose, and is a component of the cell walls of several species of red algae that are usually harvested in eastern Asia and California. Dissolved in boiling water and cooled, laboratory agar looks gelatinous. Although agar's chief use is as a culture medium for various microorganisms, particularly for bacteria, its other less well-known uses include serving as a thickening for soups and sauces, in jellies and ice cream, in cosmetics, for clarifying beverages, and for sizing fabrics.

One might ask why agar, as opposed to regular gelatin (like that found in Jello), is used for culturing bacteria. The answer is agar, unlike gelatin, won't be degraded (eaten) by bacteria. Also, agar is firmer and stronger than gelatin. It's still possible, however, to use gelatin as a culture medium for bacteria if agar is unavailable.

The Difco & BBL Manual gives more details about agar and its usage:

Agar is a phycocolloid extracted from a group of red-purple marine algae (Class Rhodophyceae) including Gelidium, Pterocladia and Gracilaria. Gelidium is the preferred source for agars. Impurities, debris, minerals and pigment are reduced to specified levels during manufacture.

Agar is a gel at room temperature, remaining firm at temperature as high as 65°C. Agar melts at approximately 85°C, a different temperature from that at which it solidifies, 32-40°C. This property is known as hysteresis. Agar is generally resistant to shear forces; however, different agars may have different gel strengths or degrees of stiffness.

Agar is typically used in a final concentration of 1-2% for solidifying culture media. Smaller quantities (0.05-0.5%) are used in media for motility studies (0.5% w/v) and for growth of anaerobes (0.1%) and microaerophiles.

Specifications for bacteriological grade agar include good clarity, controlled gelation temperature, controlled melting temperature, good diffusion characteristics, absence of toxic bacterial inhibitors and relative absence of metabolically useful minerals and compounds

[EBOOK] Nutrient Agar for microbiology

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