XLT4 Agar Base with Tergitol 4 supplement, was developed in 1990 by Miller and Tate. It is a highly selective medium for isolating Salmonella from competing bacteria such as Proteus. They reported the isolation of non-typhi Salmonella from chicken and farm environmental drag-swab samples from heavily contaminated samples. XLT4 Agar can be used clinically to screen stool samples for non-typhoid Salmonella. This medium allows the optimum growth of Salmonella. Differentiation of Salmonella from other organisms in this medium is based on the fermentation of carbohydrates (Lactose, Xylose, Sucrose) with the resulting production of hydrogen sulfide. H2S production is detected by the reaction of the iron salt, colonies appearing black or black-centered. Sodium thiosulfate and ferric ammonium citrate are the H2S indicators. The bacteria that decarboxylate the L-Lysine to cadaverine are identified by the presence of a purple-red color around the colonies due to the elevation of the pH. Phenol red is the pH indicator. Sodium thiosulfate is also added as a source of inorganic sulfur. Yeast extract and peptone are a nitrogen and amino acids source. Bacteriological agar is the solidifying agent. XLT4 Supplement (Cat. 6062) is added to inhibit the growth of non-Salmonella organisms. Typical Salmonella colonies (H2S-positive) appear black or black-centered with a yellow halo after 18-48 hours of incubation at a temperature of 35Â±2 ºC. Upon continued incubation, the colonies become entirely black or pink to red with black centers. Colonies of H2S-negative Salmonella strains appear pink-yellow. Most Citrobacter colonies are yellow without evidence of blackening. The growth of Enterobacter aerogenes and Escherichia coli is markedly inhibited; colonies that grow in this medium appear yellow without evidence of blackening. The growth of Proteus, Pseudomonas and Yersinia enterocolitica is markedly to completely inhibited. Shigella species are partially inhibited and colonies appear red.
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