What is the role of lactose in EMB?

In Eosin Methylene Blue (EMB) agar, lactose serves as a fermentable carbohydrate that allows for the differentiation of lactose-fermenting bacteria from non-fermenters. Lactose fermenters, such as Escherichia coli, produce acid during fermentation, resulting in a color change in the medium due to the pH indicators eosin and methylene blue. This leads to the formation of dark purple colonies, while non-fermenters typically produce colorless or light-colored colonies. Thus, lactose in EMB is crucial for identifying and isolating lactose-fermenting enteric bacteria.