Kligler Iron Agar is a differential medium for the identification of enteric bacteria based on the fermentation of dextrose, lactose and H2S production. This medium allows the differentiation of Gram-negative bacilli by their ability to ferment dextrose or lactose, and to produce hydrogen sulfide. The enzymatic digestion of casein and the enzymatic digestion of animal tissues provide the nitrogen, carbon and vitamins necessary for the growth of the organism. Ferric ammonium citrate and sodium thiosulphate are indicators of hydrogen sulphide production. Phenol red is the indicator of pH. Sodium chloride maintains the osmotic balance of the medium. Agar is the solidifying agent. Fermentation of dextrose leads to the production of acid, which changes the indicator from red to yellow. As there is little sugar, i.e. dextrose, the production of acid is very limited and therefore a reoxidation of the indicator is produced on the surface of the medium, and the indicator remains red. However, when lactose is fermented, the large amount of acid produced avoids reoxidation and therefore the whole medium turns yellow.