XLD agar is a selective agar that enables the identification of Salmonella species over other bacteria. This medium relies on three indicator systems:

1. Degradation of xylose, lactose, and sucrose to acid causes phenol red to change its color to yellow.
2. Production of hydrogen sulfide is indicated by thiosulfate and iron (III) salt, which react to form a precipitate of black iron sulfide in the colonies.
3. The appearance of a purple coloration around the colonies due to an increase in pH indicates bacteria that decarboxylate lysine to cadaverine.

These reactions can proceed simultaneously or successively and may cause the pH indicator to exhibit various shades of color, or it may change its color from yellow to red on prolonged incubation. The culture medium is moderately selective thus, the growth of unwanted organisms is suppressed, while sought bacteria can be tentatively grouped by reading the net effect of carbohydrate dissimilation, lysine decarboxylation, and the formation of hydrogen sulfide leading to black colonies. Agar is the solidifying agent.

ReadyPlate^™ XLD Agar plates are suitable for isolation and differentiation of Salmonella from food and animal feed, water, and other materials.

READYPLATE is a trademark of Merck KGaA, Darmstadt, Germany