Eosin methylene blue (EMB) is a selective stain for Gram-negative bacteria. It is a blend of two stains, eosin and methylene blue. A common application of this stain is in the preparation of EMB agar, a differential microbiological medium which inhibits the growth of Gram-positive bacteria and provides a color indicator distinguishing between those organisms that ferment lactose versus those that do not. Organisms which ferment lactose display "nucleated colonies" -- colonies with dark centers.
On EMB if E.coli is grown it will give off a distinctive metallic green sheen (due to acid production). It is the only bacterium that reacts this way to EMB so it can be quickly used to identify E. coli
