Eosin methylene blue

E. coli on EMB agar

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 for Gram positive bacteria and bile salt which is toxic for Gram negative bacteria other than coliforms . 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. A common application of this stain is in the preparation of EMB agar, 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]

This medium is important in medical laboratories by distinguishing pathogenic microbes in a short period of time.[4]

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

  1. 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.
  2. "Biochemical Tests Explanation"
  3. Differential Media (Levine's Formulation)
  4. 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.
  5. EMB Agar Growth Examples

External links

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