INTENDED USE
- TraceMedia™ XLD Agar is a culture medium used for the isolation of Salmonella and Shigella from pharmaceutical, food, and water samples.
PRINCIPLE
- TraceMedia™ XLD Agar was developed by Taylor to enhance the recovery of Salmonella and Shigella.
- Sodium desoxycholate is an inhibitory substance for Gram-positive bacteria.
- Xylose is fermented by most enteric pathogens, except Shigella, enabling the differentiation of Shigella from other bacterial species. After xylose is depleted, Salmonella decarboxylates lysine (via lysine decarboxylase) into cadaverine, raising the pH. Salmonella bacteria then appear similar to Shigella in an alkaline environment.
- The formed colonies appear red due to the presence of the pH indicator phenol red.
- The addition of lactose and sucrose to the medium allows enteric bacteria to decarboxylate lysine and produce excessive acid, causing the indicator to turn yellow, facilitating their differentiation.
- In the basic medium, H2S-producing substances reduce citrate, leading to the formation of black centers within colonies.
- XLD agar serves as both a selective and differential medium for Salmonella and Shigella. It contains yeast extract as a nutrient and vitamin source. Additionally, sodium deoxycholate is used as a selective agent, inhibiting Gram-positive bacteria. Xylose is incorporated into the medium as it is fermented by all enteric bacteria except Shigella, and this characteristic allows for the differentiation of Shigella species.
- Lysine is included to aid in distinguishing Salmonella from other non-pathogenic bacteria. In the absence of lysine, salmonellae will rapidly ferment xylose and cannot be differentiated from other species. After salmonellae deplete the supply of xylose, lysine is attacked via the enzyme lysine decarboxylase, leading to alkaline reversal, which precedes Shigella’s reaction.
- To prevent alkaline reversal by lysine-positive coliform bacteria, lactose and sucrose are added to produce an excess of acid. The breakdown of xylose, lactose, and sucrose into acid causes the phenol red indicator to turn from red to yellow. Bacteria that decarboxylate lysine to form cadaverine can be recognized by the presence of red color around the colony due to elevated pH. These reactions can occur simultaneously or sequentially, resulting in the pH indicator displaying various color morphologies or changing its color from yellow to red with extended incubation.
- To further enhance the differentiating capability of this formula, an H2S-indicator system, including sodium thiosulfate and ferric ammonium citrate, is included for the detection of hydrogen sulfide production, leading to the formation of colonies with black centers.