Xylose lysine deoxycholate agar (XLD agar) is aselectivegrowth medium used in the isolation ofSalmonella andShigella species from clinical samples and from food.[1][2] The agar was developed byWelton Taylor in 1965.[3] It has apH of approximately 7.4, leaving it with a bright pink or red appearance due to the indicator phenol red. Sugarfermentation lowers the pH and the phenol red indicator registers this by changing to yellow. Most gut bacteria, includingSalmonella, can ferment the sugarxylose to produce acid;Shigella colonies cannot do this and therefore remain red. After exhausting the xylose supplySalmonella colonies will decarboxylate lysine, increasing the pH once again to alkaline and mimicking the redShigella colonies. Salmonellae metabolise thiosulfate to producehydrogen sulfide, which leads to the formation of colonies with black centers and allows them to be differentiated from the similarly colouredShigella colonies.
Otherenterobacteria such asE. coli will ferment thelactose present in the medium to an extent that will prevent pH reversion bydecarboxylation and acidify the medium, turning it yellow.
XLD agar contains:
| Yeast extract | 3g/l |
| L-lysine | 5g/l |
| Xylose | 3.75g/l |
| Lactose | 7.5g/l |
| Sucrose | 7.5g/l |
| Sodiumdeoxycholate | 1g/l |
| Sodium chloride | 5g/l |
| Sodium thiosulfate | 6.8g/l |
| Ferricammoniumcitrate | 0.8g/l |
| Phenol red | 0.08g/l |
| Agar | 12.5g/l |
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