The characteristics of agar that makes it a useful solidifying agent for growth media is it is unable to be metabolized by most microorganisms. Agar is a common solidifying agent for a culture medium; a complex polysaccharide derived from a marine alga - long been used as a thickener in foods such as jellies and ice cream. Agar has some very important properties that make it valuable to microbiology, and no satisfactory substitute has ever been found. A small number of microorganisms can damage agar so it remains solid. Also, agar liquefies at about 100°C which is the boiling point of water and at sea level remains liquid until the temperature drops to about 40°C. For laboratory use, agar is held in water baths at about 50°C. At this temperature, it does not injure most bacteria when it is poured over them. As soon as the agar has hardened it can be keep warm at high temperature approaching 100°C before it again dissolves and is particularly valuable when thermophilic bacteria are grownup.
