XLD agar

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Xylose lysine deoxycholate agar (XLD agar) is a selective growth medium used in the isolation of Salmonella and Shigella species from clinical samples and from food. 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 metabolise thiosulfate to produce hydrogen sulfide, which leads to the formation of colonies with black centers and allows them to be differentiated from the similarly coloured Shigella colonies.

  • Salmonella species: red colonies, some with black centers.
  • Shigella species: red colonies.
  • Coliforms: yellow to orange colonies.
  • Pseudomonas aeruginosa: pink, flat, rough colonies.

XLD agar contains:

Yeast extract 3g/l
L-Lysine 5g/l
Xylose 3.75g/l
Lactose 7.5g/l
Sucrose 7.5g/l
Sodium deoxycholate 1g/l
Sodium chloride 5g/l
Sodium thiosulfate 6.8g/l
Ferric ammonium citrate 0.8mg/l
Phenol red 0.08g/l
Agar 12.5g/l

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