XLT4 Agar

Xylose is fermented by enteropathogenic bacteria, with the exception of Shigella, which are therefore differentiated from other bacteria. After having exhausted the xylose, Salmonella decarboxylate lysine (via lysine decarboxylase) to cadaverine, which provokes an increase in the pH. In an alkaline (basic) environment Salmonella forms red colonies in the presence of the pH indicator, phenol red.

Black colonies, due to the appearance of iron sulfide in the colony center, are formed through the reduction of ferric ammonium citrate by pathogenic hydrogen sulfide producers.
The medium contains two additional sugars, lactose and sucrose. Fermentation of either or both sugars results in acidification of the medium and leads to the formation of yellow colonies in the presence of phenol red indicator.

Non-pathogenic strains that do not decarboxylase lysine produce an acidification from the sugar fermentation. The resulting decrease in pH prevents the blackening of the colonies.

Tergitol 4 is the commercial name of a 26-28% solution of an anionic detergent, 7-ethyl 2-methyl 4-undecyl sulfate, in a sodium salt form. It inhibits contaminating Gram-positive flora and numerous Gram-negative strains, notably Proteus.