Simmons Citrate Agar can be used to distinguish between E. coli and Enterobacter aerogenes and to help differentiate members of the Salmonella group. This medium is based on the use of citrate as a carbon source. Organisms capable of using citrate grow well on the surface of the medium. The alkalinization of the medium turns it from green to blue. If the organism cannot use citrate, growth is weak or nil and the medium remains unchanged. Ammonium dihydrogen phosphate is the only source of nitrogen. Dipotassium phosphate acts as a buffer. Sodium chloride maintains the osmotic balance of the medium. Sodium citrate is the only source of carbon in this medium. Magnesium sulfate is a cofactor for various metabolic reactions. Bacteriological agar is the solidifying agent. Organisms capable of using dihydrogen phosphate and ammonium citrate grow without restriction on this medium. If citrate can be used, the microbe will accumulate alkaline or basic by-products.