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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. Other Enterobacteria such as ''E. coli'' will ferment the lactose and sucrose present in the medium to an extent that will prevent pH reversion by decarboxylation and acidify the medium turning it yellow. *''Salmonella'' species: red colonies, some with black centers. The agar itself will turn red due to the presence of Salmonella type colonies. *''Shigella'' species: red colonies. *Coliforms: yellow to orange colonies. *''Pseudomonas aeruginosa'': pink, flat, rough colonies. This type of colony can be easily mistaken for Salmonella due to the color similarities. XLD agar contains: ==See also== *R2a agar *MRS agar *Agar plate 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「XLD agar」の詳細全文を読む スポンサード リンク
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