Lysine Iron Agar was originally designed by Edwards and Fife for the examination of colonies which blackened bismuth sulfite agar. Both Arizona and Salmonella blacken bismuth sulfite; however, some strains of Arizona actively ferment lactose which in turn suppresses the production of H2S in triple sugar iron agar. Lysine Iron Agar was designed to eliminate this problem. The incorporation of lysine is important since most species of both Salmonella and Arizona produce lysine decarboxylase. The authors reported that organisms which produce lysine decarboxylase raise the pH of the medium. Lysine Iron Agar is useful in detecting members of the Proteus and Providencia groups as these organisms produce a distinct red slant over an alkaline butt. This medium is not intended to replace or be a substitute for triple sugar iron agar. Sodium thiosulfate is the source of hydrogen sulfide and the ferric salt is the indicator. The carbohydrate dextrose is incorporated in the medium in a 0.1% concentration. All Gram negative enteric bacilli uniformly ferment dextrose. Some enteric organisms are capable of producing gas from dextrose. Gas production is shown by bubbles or breaks in the medium.
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