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Xld agar results9/10/2023 Salmonellae rapidly ferment xylose and exhaust the supply. Lysine is included to differentiate the Salmonella group from the non-pathogens. Sodium chloride maintains the osmotic balance of the medium. Though the sugars xylose, lactose and sucrose provide sources of fermentable carbohydrates, xylose is mainly incorporated into the medium since it is not fermented by Shigellae but practically by all enteric species. The medium contains yeast extract, which provides nitrogen and vitamins required for growth. The media formulation does not allow the overgrowth of other organisms over Salmonella and Shigella. XLD Agar was formulated by Taylor for the isolation and differentiation of enteric pathogens including Salmonella Typhi from other Salmonella species from foods, water and dairy products. XLD Agar has been recommended for the identification of Enterobacteriaceae. Xylose Lysine Deoxycholate (XLD) agar is not intended for use in the diagnosis of diseases or other hunan condition. XLD Agar relies on Xylose fermentation, lysine decarboxylation and production of hydrogen sulfide for the primary differentiation of shigellae and salmonellae from non-pathogenic bacteria. 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. After exhausting the xylose supply Salmonella colonies will decarboxylate lysine, increasing the pH once again to alkaline and mimicking the red Shigella colonies. Most gut bacteria, including Salmonella, can ferment the sugar xylose to produce acid Shigella colonies cannot do this and therefore remain red. Sugar fermentation lowers the pH and the phenol red indicator registers this by changing to yellow. Xylose Lysine Deoxycholate agar ( XLD agar) is a selective growth medium used for the isolation and differentiation of Enterobacter, especially Salmonella and Shigella species from food, environmental samples and clinical specimens.The agar was developed by Welton Taylor in 1965. It has a pH of approximately 7.4, leaving it with a bright pink or red appearance due to the indicator phenol red.
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