Most enteric bacteria including Salmonella spp., can ferment xylose to produce acid. Sodium chloride maintains the osmotic balance. Sodium deoxycholate, sodium thiosulphate and ferric ammonium citrate are selective agents. Lactose, sucrose and xylose are fermentable carbohydrates. The yeast extract is the source of the required nitrogen, carbon and vitamins. This medium was found to be satisfactory for the isolation of Shigella and Providencia spp., as well as proving to be an effective differential media. The addition of sodium thiosulfate, ferric ammonium citrate, and sodium deoxycholate created the more selective medium, XLD agar. Pharm.Xylose Lysine Deoxycholate (XLD) agar is used for the isolation and detection of Salmonella and Shigella spp.ĭeveloped by Taylor, xylose lysine agar base was used for isolating and differentiating Gram-negative enteric bacilli. ![]() ![]() Scharlau - Xylose Lysine Deoxycholate Agar (Eur.Handbook of Culture Media for Food Microbiology- Xylose lysine deoxycholate (XLD) agar. ![]() BD XLD Agar (Xylose-Lysine-Desoxycholate Agar).Color Atlas of Medical Bacteriology 4ed.Gallinarum can form red colonies without H2S, thus resembling Shigella ⚡ Some species of Shigella (1%) ferment lactose and produce atypical yellow colonies ⚡ Prolonged incubation beyond the recommended times is not prescribed as false positives may occur due to alkaline reversion of normally acidic colonies the color disappears after 24 hours, so observations should be made between 18 and 24 hours ⚡ Incubation for more than 48 hours may lead to false positive results. coli Partial inhibition Yellow colonies Enterobacter / Klebsiella Partial inhibition Yellow colonies / Yellow and mucoid colonies Proteus Red to yellow colonies, some strains of Proteus will give colonies with a black center Pseudomonas Partial inhibition Red colonies "Shigella-like" Enterococcus Inhibited / Gram-positive bacteria Inhibited / et Salmonella H2S negative, Providencia Good Red colonies E. ⚡ The inclusion of an H2S indicator system composed of sodium thiosulphate and ferric ammonium citrate improves the differentiation capacity of the preparation: this makes it possible to visualize the production of hydrogen sulphide, which causes the formation of colonies with black centers.īacteria Growth Results Salmonella H2S positive, Edwardsiella Good Red colonies with black center Shigella spp. ⚡ Yellow colonies are also observed for lysine-negative organisms, such as Proteus species. These organisms include Escherichia, Klebsiella, Serratia, Citrobacter koseri, Yersinia enterocolitica. Other uninhibited organisms capable of fermenting lactose or sucrose will produce yellow colonies due to continued acid production, the high level of acid produced prevents the pH from returning to an alkaline value, and d 'a resulting change in pH. However, the presence of Salmonella and Edwardsiella spp is differentiated from that of shigella by an indicator of hydrogen sulfide. ![]() Organisms capable of fermenting xylose only, such as Salmonella, will deplete the xylose supply and start using lysine thereby changing the pH to alkaline and the color returning to red. Sodium thiosulfate and ferric ammonium serve as indicators of hydrogen sulfide (H2S) production under alkaline conditionsĮ.coli on XLD unable to utilize carbohydrates, such as Shigella and Edwardsiella, will not produce any significant changes and therefore colonies and medium will remain red after incubation.This alkaline pH brings back the red color of the medium. The decarboxylation of lysine, in the absence of lactose and sucrose fermentation, results in a return to an alkaline pH.Fermentation of sugars brings the pH of the medium to an acidic state and the color of the medium turns yellow Xylose, lactose and sucrose, together with phenol red, are fermentable carbohydrates.◈ Differentiation : Is done by three indicator systems ◈ Selectivity : The selective agent is sodium deoxycholate, a bile salt, which inhibits the growth of gram-positive organisms. The three added carbohydrate sources are present in different concentrations, xylose is limited while lactose and sucrose are considered inexhaustible during the prescribed incubation period. Xylose lysine deoxycholate (XLD) agar is a selective, differential and indicator medium.
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