XLD Agar (ISO 6579-1) RTU

Sodium desoxycholate inhibits contaminating Gram-positive flora.
Xylose is fermented by practically all enteropathogenic bacteria, except for Shigella which are thus differentiated from the other species. After exhausting xylose, Salmonella decarboxylate lysine (via lysine decarboxylase) to cadaverine, causing the pH to rise. Colonies of salmonellae resemble those of shigellae in the medium having become basic.

The colonies formed are red in the presence of the indicator, phenol red.
The addition of lactose and sucrose to the medium enable coliform bacteria to decarboxylate lysine and thereby produce excess acidity, making the indicator turn yellow, favoring their differentiation.

In basic medium, pathogenic H₂S-producers reduce ferric ammonium citrate and cause a blackening due to the production of iron sulfide at the center of the colonies. Non-pathogenic bacteria which do not decarboxylate lysine acidify the medium, a result of sugar fermentation. The pH decrease prevents the colonies from blackening.