Eosin Methylene Blue Agar
Eosin Methylene Blue (EMB) agar is both selective and differential. It contains the dyes eosin and methylene blue, which inhibit the growth of gram-positive bacteria and therefore select for gram-negative bacteria. It also contains the carbohydrate lactose, which allows differentiation of gram-negative bacteria based on their ability to ferment lactose.
Quadrant 1: Growth on the plate indicates the organism, Escherichia coli, is not inhibited by eosin and methylene blue and is a gram-negative bacterium. The green metallic sheen indicates E. coli is able to ferment lactose to produce strong acid end-products.
Quadrant 2: Growth on the plate indicates the organism, Pseudomonas aeruginosa, is not inhibited by eosin and methylene blue and is a gram-negative bacterium. The absence of color in the bacterial growth indicates P. aeruginosa is unable to ferment lactose.
Quadrant 3: Growth on the plate indicates the organism, Enterobacter aerogenes, is not inhibited by eosin and methylene blue and is a gram-negative bacterium. The pink color of the bacterial growth indicates E. aerogenes is able to ferment lactose to produce weak acid end-products.
Quadrant 4: Absence of growth indicates the organism, Staphylococcus aureus, is inhibited by eosin and methylene blue and is a gram-positive bacterium.