View source for XLD agar

{{SI}}


'''[[Xylose]] [[lysine]] [[deoxycholate]] [[agar]]''' (XLD agar) is a [[wikt:selective|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.
*[[Coliform]]s: yellow to orange colonies.
*''[[Pseudomonas aeruginosa]]'': pink, flat, rough colonies.

XLD agar contains:

{| class="wikitable"
| [[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
|}

==See Also==
*[[R2a agar]]
*[[MRS agar]]
*[[agar plate]]


[[Category:Microbiological media]]
[[Category:Bacteriology]]



{{WikiDoc Help Menu}}
{{WikiDoc Sources}}