Iron Agar (Lyngby) is used for the enumeration of H2S producing bacteria. Bacteriological peptone provides nitrogen, vitamins, minerals, and amino acids essential for growth. Yeast extract is a source of vitamins, particularly of the B-group. Sodium chloride supplies essential electrolytes for transport and osmotic balance. Sodium thiosulfate is reduced to hydrogen sulphide, which reacts with the iron salt to give the black iron sulphide. L-Cysteine is added to detect the production of H2S of bacteria that do not produce hydrogen sulphide from thiosulfate but only from the sulphur containing amino acid. Bacteriological agar is the solidifying agent. Bacteria capable of forming H2S from either source of sulphur would appear as black colonies.