Xylose Lysine Deoxycholate (XLD) Agar

Xylose Lysine Deoxycholate (XLD) Agar is used for the selective isolation of Salmonella and Shigella. Sugar fermentation lowers the pH below 7.4 and is indicated by a color change from red to yellow. Shigella colonies cannot convert the xylose to acid, therefore the agar will remain red. Salmonella colonies are able to perform this conversion, and will thus change the agar to yellow. After consuming all available xylose, Salmonella will decarboxylate lysine and raise the pH, changing the color back to red. Salmonella colonies can be identified by their black center, from the metabolisation of thiosulfate to hydrogen sulfide.