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Eosin-Methylene Blue (EMB) agar is a selective & differential medium used to isolate Gram-negative enteric bacteria and distinguish between them. EMB contains 2 dyes--Eosin and methylene blue--which act as selective agents to inhibit Gram-positive bacteria but allow for the growth of Gram-negative bacteria.
EMB also contains the sugars lactose and sucrose--and it distinguishes or differentiates between lactose or sucrose fermenters from non-fermenters. The fermenters form colored colonies and the non-fermenters form colorless colonies, similar to the MacConkey agar.
Figure 1: Eosin-methylene blue ( EMB) agar plate inoculated with Escherichia coli (a Gram-negative coliform bacterium) showing good growth of dark blue-black colonies with metallic green sheen indicating vigorous fermentation of lactose and acid production which precipitates the green metallic pigment. Image by Naowarat Cheeptham, Thompson Rivers University, Kamloops, BC, Canada.