Eosin Methylene Blue Agar (EMB) is a selective and differential culture medium used primarily for isolating and identifying Gram-negative bacteria, particularly Escherichia coli and Enterobacter species. The medium contains eosin Y and methylene blue dyes, which inhibit the growth of Gram-positive bacteria and provide color indicators for lactose fermentation. When lactose-fermenting bacteria grow on EMB, they produce acid, leading to a color change in the medium. E. coli typically forms dark purple colonies with a metallic green sheen, while other lactose fermenters may appear pink. Non-lactose fermenters remain colorless, allowing for easy differentiation.