Eosin methylene blue

Eosin mythylene 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-postive bacteria and provides a color indicator distinguishing between those organsisms that metabolize lactose versus those that do not.

Reference

http://medic.med.uth.tmc.edu/path/emb.htm