Agar is ideal for the screening of samples containing mixed flora and suspected of harbouring enteric pathogens e.g. The recovery of salmonellae and shigellae is not obscured by profuse growth of other species 3 therefore X.L.D. Agar and these other media have been recorded in the literature 4,2,5,6,7,8,9,10. Many favourable comparisons between X.L.D. Eosin Methylene Blue, Salmonella-Shigella and Bismuth Sulphite agars, which tend to suppress the growth of shigellae. Agar exceeds that of the traditional plating media e.g. The sensitivity and selectivity of X.L.D.
This has been due to inadequate isolation media 3. has previously been neglected despite the high incidence of shigellosis. The concentration used allows for the inhibition of coliforms without decreasing the ability to support shigellae and salmonellae. Sodium desoxycholate is incorporated as an inhibitor in the medium. The acid level also prevents blackening by these micro-organisms until after the 18-24 hour examination for pathogens. The high acid level produced by fermentation of lactose and sucrose, prevents lysine-positive coliforms from reverting the pH to an alkaline value, and non-pathogenic hydrogen sulphide producers do not decarboxylate lysine. is differentiated from that of shigellae by a hydrogen sulphide indicator. However, the presence of Salmonella and Edwardsiella spp. Salmonellae exhaust the xylose and decarboxylate the lysine, thus altering the pH to alkaline and mimicking the Shigella reaction. are differentiated from non-pathogenic xylose fermenters by the incorporation of lysine in the medium. may be identified by a negative reaction. Xylose is thus included in the medium so that Shigella spp. Rapid xylose fermentation is almost universal amongst enteric bacteria, except for members of the Shigella, Providencia and Edwardsiella genera 1. It relies on xylose fermentation, lysine decarboxylation and production of hydrogen sulphide for the primary differentiation of shigellae and salmonellae from non-pathogenic bacteria. It has since been found to be a satisfactory medium for the isolation and presumptive identification of both salmonellae and shigellae 2. Xylose-Lysine-Desoxycholate Agar was originally formulated by Taylor 1 for the isolation and identification of shigellae from stool specimens. It is important to avoid preparing large volumes which will cause prolonged heating. Pour into sterile Petri dishes as soon as the medium has cooled. Transfer immediately to a water bath at 50☌. Heat with frequent agitation until the medium boils. Suspend 53g in 1 litre of distilled water. * Adjusted as required to meet performance standards A selective medium for the isolation of salmonellae and shigellae from clinical specimens and foods.
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