Eosin Methylene Blue (EMB) Agar
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Eosin methylene blue agar (EMB) agar is a both a selective medium and a differential medium. EMB agar is used to isolate pathogenic fecal coliforms.
Eosin Y and methylene blue are pH indicators that form a dark purple precipitate under acidic conditions (at low pH). Eosin Y and methylene blue (EMB) agar is selective, because it inhibits the growth of Gram-positive bacteria. |
Sucrose and lactose are added in EMB agar to distinguish Gram-Negative species of bacteria that are able to ferment lactose from those bacterial species that cannot. Fermentation creates an acidic byproduct that will cause a color change due to the presence of the pH indicators (Eosin Y and Methylene Blue). Species that are able to ferment lactose, therefore, will produce dark purple, black or green colonies on the EMB agar plate. Bacteria that are able to ferment large amounts of lactose quickly, such as Escherichia coli, will produce more acidic byproduct which often produces a dark metallic green color. Slow or weak fermenters will produce pink colonies. Species that cannot ferment eith sucrose of lactose, will produce colorless colonies.
Fecal Coliforms Show up as Dark Colonies on EMB Agar.
Fecal Coliforms Show up as Dark Colonies on EMB Agar.