Principle
- Xylose lysine deoxycholate agar is a differential medium, slightly selective for the detection of pathogenic enterobacteria, especially Salmonella and Shigella.
- Media 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 carbohydrates. Lysine is also added to differentiate Salmonella, which undergoes decarboxylation to cadaverine.
- Phenol red is a pH indicator. Sodium desoxycholate inhibits the 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 the medium, as the concentration of xylose declined, the Salmonella organism’s decarboxylate lysine caused its version to alkaline conditions.
- The resulting 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 an indicator of hydrogen sulfide production under alkaline conditions.
- The hydrogen sulfide producers are detected by darker colonies, due to the ferric sulfate precipitate.
- Due to xylose, lactose or sucrose fermentation produces acidification, and lysine decarboxylation to cadaverine causes alkalization, and consequently the phenol red indicator turns 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 the basis of ferrous sulfur precipitation. The Salmonella colonies show red colored with black centers and Shigella are red colored without black centers.
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