Agar

Previously gelatin was used as a solidifying agent in a media. But it has two disadvantages; firstly, it melts at incubator temperature i.e. 37°C and secondly, it is attacked and decomposed by many bacteria. Koch substituted gelatin with agar to solidify nutrient media. Agar, isolated from red algae, is a sulfated polymer composed mainly of D-galactose, 3,6-anhydro-L-galactose, and D-glucuronic acid. Generally 1.0 to 2.0 % of agar is added to liquid media to solidify it. It dissolves at a temperature of boiling water and solidifies when temperature drops to about 45°C. It can overcome the two disadvantages of gelatin i.e. it melts at a temperature of 80 to 90°C and not at incubator temperature; and it is not degraded by most of the microorganisms. An agar plate is used to provide a growth medium to microorganisms.