![]() Salmonella: H 2S positive Red colonies with black centers.for 18-24 hours.Ĭolony Characteristics of various organisms in XLD Agar Incubate plates aerobically at 35-37✬.Streak for isolation with a sterile loop.If the specimen to be cultured is on a swab, roll the swab over a small area of the agar surface.Inoculate and streak the specimen as soon as possible after collection.Allow the plates to warm at 37☌ or to room temperature, and the agar surface to dry before inoculating.Test procedure ( specimen/organism inoculation) Product is light and temperature sensitive protect from light, excessive heat, moisture, and freezing.Media should not be used if there are any signs of deterioration (shrinking, cracking, or discoloration), contamination.Store at 2-8✬ and away from direct light.It is advisable not to prepare large volumes that will require prolonged heating, thereby producing precipitate.After cooling, pour into sterile Petri plates.Transfer immediately to a water bath at 50☌.Heat with frequent agitation until the medium boils.Oxoid says 53 gm in 1 litre while Hardy Diagnostics 56.93 and Himedia 56.68. Note:The amount of XLD agar varies manufacturer to manufacture e.g. Suspend 56.68 grams dehydrated powder XLD agar in 1000 ml distilled or deionized or purified water.Escherichia coli, Citrobacterand Proteusspecies. The non-pathogenic H2S producers do not decarboxylase lysine therefore, the acid reaction produced by them prevents the blackening of the colonies.Organisms which ferment xylose, are lysine decarboxylase-negative, and do not ferment lactose or sucrose cause an acid pH in the medium, and form yellow colonies e.g. To add to the differentiating ability of the formulation, an H2S indicator system, consisting of sodium thiosulphate and ferric ammonium citrate, is included for the visualization of hydrogen sulphide produced, resulting in the formation of colonies with black centers. Subsequently lysine is decarboxylated by the enzyme lysine decarboxylase to form amines with reversion to an alkaline pH that mimics the Shigella reaction. ![]() 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 Shigellacolonies. After exhausting the xylose supply Salmonellacolonies will decarboxylate lysine, increasing the pH once again to alkaline and mimicking the red Shigellacolonies. Most gut bacteria, including Salmonella, can ferment the sugar xylose to produce acid Shigellacolonies 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 Salmonellaand Shigellaspecies 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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