XLD

Xylose Lysine Deoxycholate (XLD) agar is a selective growth medium used for the isolation of Salmonella and Shigella species from clinical samples and from food. XLD Agar was developed by Taylor for the differentiation, isolation, and identification of enteric pathogens, and to support the growth of more fastidious enteric organisms. XLD Agar was specially designed to allow the growth of Shigella species and is a proven medium for the isolation of this organism. It has also been found to be an excellent medium for isolating Salmonella species as well. It has a pH of approximately 7.4, leaving it with a bright pink or red appearance due to the indicator phenol red. Sugar fermentation lowers the pH and the phenol red indicator registers this by changing to yellow. Most gut bacteria, including Salmonella, can ferment the sugar xylose to produce acid; Shigella colonies cannot do this and therefore remain red. After exhausting the xylose supply Salmonella colonies will decarboxylate lysine, increasing the pH once again to alkaline and mimicking the red Shigella colonies. Salmonellae metabolize thiosulfate to produce hydrogen sulfide, which leads to the formation of colonies with black centers and allows them to be differentiated from the similarly colored Shigella colonies.