Lysine Iron Agar

 Principle

• Lysine iron agar is composed of peptone, yeast extract, dextrose, L-lysine hydrochloride, ferric ammonium citrate, sodium thiosulphate, bromocresol purple, and agar.
• Peptone provides nitrogen and amino acids and yeast extract serves as a source of carbon, nitrogen, vitamins, and other growth factors required for the growth of microorganisms.
• Dextrose is a carbon and energy source. L-lysine hydrochloride is a substrate use for lysine decarboxylase and deaminase detection. Sodium thiosulfate and ferric ammonium citrate are indicators for hydrogen sulfide production.
• Bromocresol purple is a pH indicator, that gives yellow color below pH 5.2 and purple color above pH 6.8. Agar is a solidifying agent.
• The bacteria particularly enteric bacilli that produce hydrogen sulfide cause blackening of the medium due to the production of ferrous sulfides.
• The microorganisms that synthesize lysine decarboxylase produce an alkaline reaction (purple color) or neutral reaction in the butt of the medium. Organisms that deaminate the lysine cause the development of a red slant over an acid butt. 

 Use: Recommended for the differentiation of enteric organisms based on their ability to decarboxylate or deaminate lysine and the formation of hydrogen sulfide.