Eosin Methylene Blue Agar, Levine is a slightly selective and differential plating medium for the isolation of gram-negative enteric bacteria

Principles The eosin Y and methylene blue dyes in Levine EMB Agar render the medium slightly selective in that they inhibit gram-positive bacteria to a limited degree. These dyes also play a role in differentiating between lactose fermenters and lactose nonfermenters due to the presence or absence of dye uptake in the bacterial colonies. Coliforms, as lactose-fermenting organisms, are visualized as blue-black colonies, whereas colonies of Salmonella and Shigella, as lactose nonfermenters, appear colorless, transparent or amber. Some gram-positive bacteria, such as fecal streptococci, staphylococci and yeasts, will grow on this medium and usually form pinpoint colonies. A number of nonpathogenic, lactose-nonfermenting gram-negative bacteria will grow on this medium and must be distinguished from the pathogenic strains by additional biochemical tests.