Xylose is incorporated into the medium since it is fermented by practically all enterics except for the shigellae, and this property enables the differentiation of Shigella species. Lysine is included to enable the Salmonella group to be differentiated from the nonpathogens since, without lysine, salmonellae rapidly would ferment the xylose and be indistinguishable from nonpathogenic species. After the salmonellae exhaust the supply of xylose, the lysine is attacked via the enzyme, lysine decarboxylase, with reversion to an alkaline pH which mimics the Shigella reaction. To prevent similar reversion by lysinepositive coliforms, lactose and sucrose (saccharose) were added to produce acid in excess.1 To add to the differentiating ability of the formulation, an H2S indicator system, consisting of sodium thiosulfate and ferric ammonium citrate, is included for the visualization of the hydrogen sulfide produced, resulting in the formation of colonies with black centers. The nonpathogenic H2S producers do not decarboxylate lysine; therefore, the acid reaction produced by them prevents the blackening of the colonies.1 XLD Agar is both a selective and differential medium. It utilizes sodium desoxycholate as the selective agent and, therefore, it is inhibitory to gram-positive microorganisms.

It is particularly recommended for obtaining counts of enteric organisms. This medium can be rendered moderately selective for enteric pathogens, particularly Shigella, by the addition of sodium desoxycholate (2.5 g/L) to make Xylose Lysine Deoxycholate Agar.1 XL Agar Base can be made selective for Salmonella by adding 1.25 mL/L of 1% aqueous brilliant green to the base prior to autoclaving. Its use is recommended for Salmonella isolation after selenite or tetrathionate enrichment in food analysis; both coliforms and Shigella are inhibited.1 XLD Agar was developed by Taylor in order to increase the efficiency of the isolation and identification of enteric pathogens, particularly Shigella. 1 The pathogens are differentiated not only from the nonpathogenic lactose fermenters but also from many nonpathogens which do not ferment lactose or sucrose. Additionally, the medium was formulated to increase the frequency of growth of the more fastidious pathogens,1 which in other formulations have often failed to grow due to the inclusion of excessively toxic inhibitors. The results obtained in a number of clinical evaluations have supported the claim for the relatively high efficiency of XLD Agar in the primary isolation of Shigella and Salmonella. 2-6 XLD Agar is included in the USP microbial limit test for screening specimens for the presence or absence of Salmonella7 and is recommended for the testing of foods, dairy products and water.