XLT-4 Agar is a highly selective plating medium used for isolation and identification of salmonellae from clinical, environmental and food samples according to Miller and Tate1
. XLT-4 Agar has been shown to save time and reduce the costs of further identification of false presumptive positive colonies when compared to other salmonellae plating media1
- Ensures optimal growth of salmonellae while reducing false-positive suspect colonies
- Inhibits unwanted background flora with sodium tetradecylsulfate
- Allows Salmonellae to be easily distinguished: appearing as black or red colonies with a black center
The peptones and yeast extract in this formulation provide a source of amino nitrogen, along with essential nutrients and vitamins; this ensures optimal growth of salmonellae.
The selective agent is sodium tetradecylsulfate, which is an anionic surfactant. This largely inhibits or reduces the growth of unwanted background flora. The background color of the plate is red, due to the inclusion of the phenol red, which will change color due to pH changes from fermentation and decarboxylation reactions.
Differentiation on this medium is facilitated by the fermentation of xylose, lactose and sucrose as well as the decarboxylation of lysine. Salmonellae appear as black or red colonies with a black center due to their ability to reduce thiosulphate to hydrogen sulphide, which causes the colony to blacken (as there are ferric ions present). Escherichia coli grow as yellow colonies as they are able to ferment the lactose in the formulation, causing a pH drop which shifts the color of the phenol indicator from red to yellow. Other organisms, such as shigellae, do not ferment the lactose, or reduce the thiosulphate, so appear as clear red colonies with no blackening.
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