Fig. 19: Salmonella Growing on XLD Agar

Salmonella on XLD agar

Neither lactose nor sucrose are fermented so no acid is produced. The amino acid lysine is broken down (decarboxylated) producing alkaline end products which cause the phenol red in the agar to turn a deeper red. Hydrogen sulfide production causes some of the colonies to appear black.

By Nathan Reading from Halesowen, UK (Salmonella species on X.L.D. agar.) [CC BY 2.0 (], via Wikimedia Commons