![]() 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. 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. Agar is typically used in a final concentration of 1-2% for solidifying culture media. Agar is generally resistant to shear forces however, different agars may have different gel strengths or degrees of stiffness. Agar melts at approximately 85☌, a different temperature from that at which it solidifies, 32-40☌. Agar is a gel at room temperature, remaining firm at temperature as high as 65☌. Impurities, debris, minerals and pigment are reduced to specified levels during manufacture. Gelidium is the preferred source for agars. The Difco & BBL Manual gives more details about agar and its usage: (3) Agar is a phycocolloid extracted from a group of red-purple marine algae (Class Rhodophyceae) including Gelidium, Pterocladia and Gracilaria. It's still possible, however, to use gelatin as a culture medium for bacteria if agar is unavailable. Also, agar is firmer and stronger than gelatin. The answer is agar, unlike gelatin, won't be degraded (eaten) by bacteria. One might ask why agar, as opposed to regular gelatin (like that found in Jello), is used for culturing bacteria. 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. Dissolved in boiling water and cooled, laboratory agar looks gelatinous. ![]() 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. With its distinctive smell, one can easily distinguish agar from the other materials commonly found in a laboratory. ![]()
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