Eosin methylene blue
Eosin methylene blue (EMB, also known as "Levine's formulation") is a selective stain for Gram-negative bacteria.[1] EMB contains dyes that are toxic to Gram-positive bacteria. EMB is the selective and differential medium for coliforms. It is a blend of two stains, eosin and methylene blue in the ratio of 6:1. EMB is a differential microbiological medium, which slightly inhibits the growth of Gram-positive bacteria and provides a color indicator distinguishing between organisms that ferment lactose (e.g., E. coli) and those that do not (e.g., Salmonella, Shigella).[2] Organisms that ferment lactose display "nucleated colonies"—colonies with dark centers.[3]
![](../I/Coli_levine.JPG.webp)
This medium is important in medical laboratories by distinguishing pathogenic 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.
![](../I/E._coli_on_EMB_agar.png.webp)
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
- 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.