Eosin Methylene Blue

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E. coli colonies on EMB agar

Eosin methylene blue agar (short: EMB agar) is a selective medium for Enterobacteriaceae and other enterobacteria . Gram positive bacteria do not grow on this medium, whereas gram negative bacteria grow on this medium. It was developed by Holt-Harris and Teague .

Mode of action

The ingredients of the EMB medium responsible for the mode of action are on the one hand the carbohydrates lactose and sucrose , as well as the colorants eosin (yellowish) and methylene blue . These dyes largely inhibit the growth of the gram-positive accompanying flora. In the nutrient medium since two different carbohydrates are included, is there different bacterial genera among the enterobacteria differ, depending on which carbohydrates can exploit them. If not inoculated, the nutrient medium is clear and red-brown and has a pH of about 7.1.

Lactose- and sucrose-negative bacteria (such as Salmonella and Shigella ) grow as transparent, amber-colored colonies on EMB agar . Lactose-negative, but at the same time sucrose-positive bacteria (such as Proteus vulgaris ) show up as dark purple colored colonies. The lactose-positive coliforms such as B. Enterobacter and Klebsiella grow with large, slimy-looking colonies that appear pink with a dark center in transmitted light. Escherichia coli is particularly easy to distinguish from it , which also belongs to the lactose-positive coliform bacteria, but is easily recognizable on the EMB agar by the greenish metallic sheen of the colonies; the colonies also have a blue-black center in the transmitted light.

Typical composition

The nutrient medium usually consists of (data in grams per liter ):

See also

Individual evidence

  1. ↑ a b c Technical information EMB-Agar from Merck KGaA, accessed on February 11, 2013.