Eosin methylene blue (EMB, also known as "Levine's formulation") is a slightly selective stain for Gram-negative bacteria.[1] 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 slightly 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).[2] Organisms that ferment lactose display "nucleated colonies" -- colonies with dark centers.[3]
This medium is important in medical laboratories by distinguishing pathogenic microbes in a short period of time.[4]
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). Some species of Citrobacter and Enterobacter will also react this way to EMB.[5] Some Gram Positive bacteria are able to grow on EMB agar such as fecal streptococci and staphylococci.
EMB contains the following ingredients: Pancreatic digest of gelatin, Lactose, Dipotassium phosphate, Eosin Y (dye), Methylene blue (dye), Agar
There are also EMB agars that do not contain lactose
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