![]() A single medium may not be enough to recover all pathogens, so a less selective medium such as MacConkey agar should also be used.Left: Yellow Escherichia coli colonies Right: Red-pink Salmonella colonies (Some Proteus species look identical) Limitations Xylose Lysine deoxycholate (XLD) Cultures. Detection of Salmonella in non-sterile pharmaceutical products (in accordance with EP, USP) and food, water dairy products, etc.Isolation and screening of samples containing mixed flora suspected of harboring enteric pathogens, e.g., medical specimens or food products.For isolation and differentiation of Salmonella and Shigella spp from other enteric pathogens.Organism Growth and colony morphology S.typhimurium ATCC 14028 Good growth, Red colonies with a black center S.flexneri ATCC 12022 Luxuriant growth, red colonies Escherichia coli ATCC 25922 Partially inhibited, Large, flat, yellow colonies Enterococcus faecalis ATCC 29212 Partial to complete inhibition, clear pinpoint colonies Uses of XLD Agar Performance testing: Inoculate known standard strains on XLD agar plates, incubate for 18- 24 hours at 35-37☌, and observe for growth and colony characteristics.Discard the whole lot if any colonies are seen. After 48 hours, the sterility test plate should remain clear. Sterility testing: Incubate un-inoculated plates of XLD agar for 48 hours at 35-37☌ and observe any growth.Quality testing of the prepared media plates should be done by performing sterility and performance testing. After preparation, the medium is bright red to reddish-orange, trace to slightly with neutral pH (7.20-7.60). The commercially available dehydrated media should be homogeneous, free-flowing, and light pink-beige in color. Left: Red-pink black centered colonies of Salmonella typhimurium Typhimurium Yellow opaque colonies Ferment xylose but not lactose and sucrose, lysine negative, gives acid pH E.coli, Klebsiella/Enterobacter Citrobacter, Serratia and Proteus spp Yellow colonies Lactose or sucrose fermentation, lysine negative, gives acid pH Possible coliforms Sucrose positive Proteus spp Quality Control Colony characteristics on XLD Agar Basis of reaction Possible pathogens Red colonies Alkaline reaction, non-fermentation of xylose/lactose/sucrose or fermentation of xylose followed by decarboxylation of lysine Shigella spp, Providencia spp, Pseudomonas spp, and H2S non-producing Salmonella spp Red colonies with a black center Xylose positive, lysine decarboxylase positive, capable of producing H2S, thus black centered colonies in alkaline pH H2S producing Salmonella spp S. Result InterpretationĪfter incubation of the plates with test organisms, various colored colonies develop, and differentiation is based on it. 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 reaches the boiling point.Suspend 55 gm of the medium in one liter of purified water.Organisms that ferment lactose, sucrose, and xylose but are lysine decarboxylase negative cause an acid pH and produce yellow colonies. However, Salmonellae also metabolize thiosulfate to produce hydrogen sulfide, which leads to the formation of colonies with black centers and allows them to be differentiated from the similarly colored Shigella colonies. This property aids in the differentiation of Shigella spp.Īfter the xylose supply is exhausted, Salmonella spp decarboxylates lysine, increasing the pH to alkaline condition, and also produces red colonies like Shigella spp. Since Shigellaspp doesn’t utilize xylose, acidification does not occur, and red colonies are produced. This causes acidification of the medium, turning the phenol red indicator yellow. ![]() Xylose is rapidly fermented by most Gram-negative enteric bacteria, including Salmonella. Ingredients in Per liter formulations: Ingredients Amount (Gram/Liter) Lactose 7.5 gm Sucrose 7.5 gm Sodium thiosulfate 6.8 gm L-Lysine 5.0 gm Sodium chloride 5.0 gm Xylose 3.75 gm Yeast extract 3.0 gm Sodium deoxycholate 2.5 gm Ferric ammonium citrate 0.8 gm Phenol red 0.08 gm Agar 15.0 gm Principle of XLD Agar Sodium chloride maintains the osmotic balance, and agar is the solidifying agent. Sodium deoxycholate present in the medium inhibits the growth of Gram-positive organisms. Yeast extract provides sources of nitrogen, carbon, and vitamins required for organism growth. The key ingredients of XLD agar are three sugars (xylose, lactose, and sucrose ), lysine, and ferric ammonium citrate. ![]()
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