Yeast Extract provides sources of nitrogen, carbon, and vitamins required for organism growth. Xylose,
Lactose, and Sucrose, provide sources of fermentable carbohydrate. Xylose is fermented by most enteric
organisms except Shigella spp. and Providencia spp. Lysine is added to differentiate Salmonella. As Xylose is
exhausted, Salmonella spp organisms decarboxylate lysine causing a reversion to alkaline conditions.
Alkaline reversion by other lysine-positive organisms is prevented by excess acid production from
fermentation of lactose and sucrose.
Sodium Thiosulfate and Ferric Ammonium Citrate act as selective agents, allowing visualization of hydrogen
sulfide production under alkaline conditions. Sodium Deoxycholate is also a selective agent. Phenol Red is an
indicator. Sodium Chloride maintains the osmotic balance in the medium. Agar is the solidifying agent.