Eosin methylene blue
Eosin methylene blue (EMB, also known as "Levine's formulation") is a selective and differential media used for the identification of Gram-negative bacteria,[1] specifically the Enterobacteriaceae. EMB inhibits the growth of most Gram-positive bacteria. EMB is often used to confirm the presence of coliforms in a sample. It contains two dyes, eosin and methylene blue in the ratio of 6:1. EMB is a differential microbiological media, which inhibits the growth of Gram-positive bacteria and differentiates bacteria that ferment lactose (e.g., E. coli) from those that do not (e.g., Salmonella, Shigella).[2] Organisms that ferment lactose appear dark/black or green often with "nucleated colonies"—colonies with dark centers.[3] Organisms that do not ferment lactose will appear pink and often mucoid.
This culture media is important in medical laboratories by allowing the identification enteric bacteria microbes in a short period of time.[4]
- Rapid lactose fermentation produces acids, which lower the pH. This encourages dye absorption by the colonies, which are now colored purple-black.
- Lactose non-fermenters may increase the pH by deamination of proteins. This ensures that the dye is not absorbed. The colonies will be colorless.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/3/3e/E._coli_on_EMB_agar.png/220px-E._coli_on_EMB_agar.png)
On EMB if E. coli is grown it will give a distinctive metallic green sheen (due to the metachromatic properties of the dyes, E. coli movement using flagella, and strong acid end-products of fermentation). Some species of Citrobacter and Enterobacter will also react this way to EMB.[5] This medium has been specifically designed to discourage the growth of Gram-positive bacteria.[6]
EMB contains the following ingredients: peptone, lactose, dipotassium phosphate, eosin Y (dye), methylene blue (dye), and agar.
There are also EMB agars that do not contain lactose.
References
[edit]- ^ Levine, M (1918). "Differentiation of B. coli and B. aerogenes on a simplified eosin-methylene blue agar". J Infect Dis. 23: 43–47. doi:10.1086/infdis/23.1.43.
- ^ ""Biochemical Tests Explanation"". Archived from the original on 2009-02-15. Retrieved 2008-12-24.
- ^ "Differential Media (Levine's Formulation)". Archived from the original on 2008-12-11. Retrieved 2008-12-24.
- ^ Bachoon, Dave S., and Wendy A. Dustman. Microbiology Laboratory Manual. Ed. Michael Stranz. Mason, OH: Cengage Learning, 2008. Exercise 8, "Selective and Differential Media for Isolation" Print.
- ^ "EMB Agar Growth Examples". Archived from the original on 2009-01-22. Retrieved 2008-12-24.
- ^ "Eosin-Methylene Blue Agar Plates Protocol". September 29, 2007. Archived from the original on November 30, 2011.