| Xylose-lysin-desoxycholat-agar (XLD)
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| The same agar plate examined with transmitted light. |
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This medium is used for the isolation of shigellae and salmonellae from foods and clinical specimens. Shigella, Providencia and Edwardsiella do not ferment xylose, sucrose or lactose, therefore they alter the pH to alkaline and produce red colonies. Salmonella spp. ferments xylose, but at the same time decarboxylates lysine in the medium causing an alkaline pH and thus producing red colonies. The hydrogen sulphide producers, Salmonella and Edwardsiella, grow colonies with a black centre because the medium contains an iron salt. Fermentation of sucrose and/or lactose produces higher acid levels, thus preventing sucrose and/or lactose positive bacteria from reverting the pH to alkaline through decarboxylation of lysine. Non-pathogenic hydrogen sulphide-producing bacteria do not decarboxylate lysine. At the same time, the level of acid produced by fermentation prevents blackening of the colonies until after 18 or 24 hours. Sodium desoxycholate is an inhibitor which, at the concentration used in this medium, will inhibit coliforms but not salmonellae and shigellae.
The pictures belows show (from left to right) an uninoculated agar, Escherichia coli on XLD and Shigella on XLD.
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