Eosin Methylene Blue Agar, Levine

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 non-fermenters 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 non-fermenters, 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.