The medium contains yeast extract, which provides nitrogen and vitamins required for growth. Though the sugars xylose,
lactose and sucrose provide sources of fermentable carbohydrates, xylose is mainly incorporated into the medium since it is
not fermented by Shigellae but practically by all enterics. This helps in the differentiation of Shigella species. Sodium
chloride maintains the osmotic balance of the medium. Lysine is included to differentiate the Salmonella group from the
non-pathogens. Salmonellae rapidly ferment xylose and exhaust the supply. Subsequently lysine is decarboxylated by the
enzyme lysine decarboxylase to form amines with reversion to an alkaline pH that mimics the Shigella reaction. However,
to prevent this reaction by lysine-positive coliforms, lactose and sucrose are added to produce acid in excess. Degradation of
xylose, lactose and sucrose to acid causes phenol red indicator to change its colour to yellow. Bacteria that decarboxylate lysine
to cadaverine can be recognized by the appearance of a red colouration around the colonies due to an increase in pH. These
reactions can proceed simultaneously or successively, and this may cause the pH indicator to exhibit various shades of colour
or it may change its colour from yellow to red on prolonged incubation.