Levine's EMB, a blend of Eosin and Methylene Blue stains, is a fast way to discover the presence or absence of bacteria that can metabolize lactose. When lactose is broken down it produces lactic acid.When using Levine's EMB with lactose fermenting bacteria, the presence of lactic acid and the associated lowering of the pH causes the bacterial colonies to absorb the stain from the medium, producing colonies that appear to be red or purple. Hence, colored colonies contain lac+ bacteria.
Non-lactose fermenting colonies will not pick up the dyes, so lac- colonies will be colorless or very pale in appearance. E. coli colonies grown on EMB will sometimes have a green sheen over the surface of the colony.