Eosin methylene blue (EMB) agar is a differential plating agar that is used in the detection of Gram negative enterics. Eosin and methylene blue are dyes that differentiate between colonies which are able to ferment lactose from those that do not ferment this sugar. Cells that can use lactose (like many enterics) are blue black with a green metallic sheen (see image below). Colonies that cannot use lactose are amber or colorless.
In addition, these same dyes inhibit the growth of Gram positive bacteria, making this a selective media.
Plates streaked with samples taken from healthy students had colonies similar to the ones shown below. Plates streaked with samples taken from sick students had both blue black colonies with a green metallic sheen, and colorless colonies.
What does this tell you about the potential pathogen?
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