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Description

Xylose Lysine Deoxycholate agar (XLD agar) is a selective growth medium introduced by Taylor (1965) for the isolation of Salmonella and Shigella species (Taylor and schelbert, 1971). It relies on the use of selective inhibitory component sodium deoxycholate and three indicator systems, i.e. xylose, lactose and sucrose combined with phenol red. Degradation of xylose, lactose, and sucrose to acid causes phenol red to change its colour to yellow. Sodium thiosulfate and iron (III) salt react to form a precipitate of black iron sulfide in the colonies, indicating the production of hydrogen sulfide. Bacteria which decarboxylate lysine to cadaverine can be recognized by the appearance of a purple coloration around the colonies due to an increase in pH. These reactions can proceed simultaneously or successively. This causes the pH indicator to exhibit various shades of colour, or it may change its colour from yellow to red on prolonged incubation. The culture medium is weakly inhibitory to other microbes due to the presence of sodium deoxycholate.