Eosin Methylene Blue Agar

Eosin methylene blue agar (EMB) is a selective and differential medium used to isolate fecal coliforms. Eosin Y and methylene blue are pH indicator dyes which combine to form a dark purple precipitate at low pH; they also serve to inhibit the growth of most Gram positive organisms. Sucrose and lactose serve as fermentable carbohydrate sources which encourage the growth of fecal coliforms and provide a means of differentiating them.

Vigorous fermenters of lactose or sucrose will produce quantities of acid sufficient to form the dark purple dye complex. The growth of these organisms will appear dark purple to black.  Escherichia coli, a vigorous fermenter, often produces a green metallic sheen. Slow or weak fermenters will produce mucoid pink colonies.  Normally-colored or colorless colonies indicate that the organism ferments neither lactose nor sucrose and is not a fecal coliform.

Escherichia coli often produces a metallic green sheen on EMB.