XLD Agar

 

Use: Selective and differential medium for the isolation, cultivation, and differentiation of enteric bacteria, particularly Shigella sp. from both clinical and non-clinical specimens.

Description: Modification of medium developed by Taylor XLD Agar has been shown to be superior to other selective media for the isolation of Shigella. Sodium deoxycholate inhibits Gram positive organisms. Xylose is fermented slowly by Shigella and Providencia and thus demonstrate an alkaline reaction (red colonies). Salmonella ferment xylose rapidly but also decarboxylate lysine giving an alkaline reaction. Excess lactose and sucrose prevent lysine positive organisms from reverting to the alkaline reaction. H2S production by Salmonella and Arizone is indicated by black centered red colonies. Sodium thiosulfate is added as the sulfur source and ferric ammonium citrate as indicator. Phenol red is used as an acid-bade indicator with lactose and sucrose fermentation, giving yellow colonies (acid reaction).

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