Eosin methylene blue

Eosin methylene blue (EMB, also known as "Levine's formulation") is a selective stain for Gram-negative bacteria. EMB contains dyes that are toxic for Gram positive bacteria and bile salt which is toxic for Gram negative bacteria other than coliforms . EMB is the selective and differential medium for coliforms. 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). Organisms that ferment lactose display "nucleated colonies"—colonies with dark centers.