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The appearance of colonies from E. coli on Eosin Methylene Blue (EMB) agar is characterized by a green sheen with a dark purple background. EMB agar is a selective and differential medium that allows for the growth of Gram-negative bacteria while inhibiting Gram-positive organisms.
E. coli ferments lactose present in the agar, producing acid and resulting in a color change of the medium. This fermentation can cause the colonies to appear dark purple due to the precipitation of the dyes (eosin and methylene blue) in the medium. The green sheen is particularly prominent in E. coli and is a result of the buildup of acids during fermentation, which affects the surface of the colonies.
In contrast, other bacteria that grow on EMB may appear as light purple to light pink colonies or even colorless depending on their lactose fermentation activity. For example, non-lactose fermenters will produce colonies that do not alter the medium color, appearing colorless. Therefore, the distinctive combination of the green sheen and dark purple background is specifically associated with E. coli on this medium.