Products

Principle

Xylose Lysine Deoxycholate Agar is a differential medium, slightly selective for the detection of pathogenic enterobacteria, especially Salmonella and Shigella. Media is composed of xylose, lysine, lactose monohydrate, sucrose, sodium chloride, yeast extract, phenol red, sodium deoxycholate, sodium thiosulphate, ferric ammonium citrate and agar. Yeast extract provides sources of nitrogen and carbon, as well as vitamins and cofactors required for growth. Xylose, lactose and sucrose are the carbon source. Xylose is fermented by most enteric organisms except Shigella and Providencia. Sucrose and lactose provide sources of fermentable carbohydrate. Lysine is also added to differentiate Salmonella, which undergoes decarboxylation to cadaverine. Phenol red is pH indicator. Sodium deoxycholate inhibits growth of gram-positive organisms, but Shigella grows at this concentration. Sodium chloride maintains osmotic balance in the medium. Agar is a solidifying agent. In medium as the concentration of xylose is declined, the Salmonella organism’s decarboxylate lysine causing is version to alkaline conditions. The resulted alkaline reversion by lysine-positive organisms is prevented by acid produced by the fermentation of lactose and sucrose. Simultaneously, the sodium thiosulfate is metabolized to hydrogen sulfide and reacts with ferric ammonium citrate which acts as indicator of hydrogen sulfide production under alkaline conditions. The hydrogen sulfide producers are detected by darker colonies, due to the ferric sulfur precipitate. Due to xylose, lactose or sucrose fermentation produce the acidification and lysine decarboxylation to cadaverine causes alkalization and consequently the phenol red indicator turns to red. This color disappears after 24 hours, so observations must be carried out between 18 and 20 hours. The Shigella and Salmonella are differentiated on basis of ferrous sulfur precipitation. The Salmonella colonies show red colored with black centers and Shigella are red colored without black center.

Use: Recommended for the isolation and enumeration of Salmonella and Shigella from pharmaceutical products.