Eosin-methylene blue (EMB) agar is selective for gram-negative bacteria against gram-positive bacteria. In addition, EMB agar is useful in isolation and differentiation of the various gram-negative bacilli and enteric bacilli, generally known as coliforms and fecal coliforms respectively.The bacteria that ferment lactose in the medium form colored colonies, while those that do not ferment lactose appear as colorless colonies. EMB agar is used in water quality tests to distinguish coliforms and fecal coliforms that signal possible pathogenic microorganism contamination in water samples. EMB agar is also used to differentiate the organisms in the colon-typhoid-dysentery group: Escherichia coli colonies grow with a metallic sheen with a dark center, Aerobacter aerogenes colonies have a brown center and non-lactose-fermenting, gram-negative bacteria appear pink.
