- Eosin methylene blue (EMB) is a selective stain for Gram-negative bacteria.
- It is a blend of two stains, eosin and methylene blue in the ratio of 6:1.
- 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 organisms that ferment lactose (e.g., E. coli) and those that do not (e.g., Salmonella, Shigella).
- Organisms that ferment lactose display "nucleated colonies" -- colonies with dark centers.
- Lactose fermentation produces acids, which lower the pH. This encourages dye absorption by the colonies, which are now coloured purple-black.
- Lactose non-fermenters may increase the pH by deamination of proteins. This ensures that the dye is not absorbed.
- On EMB if E.coli is grown it will give a distinctive metallic green sheen (due to the metachromatic properties of the dyes, E. coli movement using flagella, and strong acid end-products of fermentation). It is one of the few bacteria that reacts this way to EMB.
