Eosin methylene blue
Eosin - methylene blue agar (short: EMB agar ) is a selective medium for Enterobacteriaceae. 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.
Operation
The responsible for the operation of the EMB medium ingredients are firstly the carbohydrates lactose and sucrose, and the dyes eosin ( yellowish) and methylene blue. These dyes largely inhibit the growth of Gram- positive microorganisms. As in the nutrient medium, two different carbohydrates are included, one can distinguish different bacterial genera among the Enterobacteriaceae, depending on which carbohydrates they can exploit. Uninoculated the nutrient medium is clear and reddish brown and has a pH of about 7.1.
Lactose and sucrose -negative bacteria (such as Salmonella and Shigella ) grow on EMB agar as a transparent, amber -colored colonies. Lactose -negative, but at the same sucrose - positive bacteria ( eg Proteus vulgaris) show up as dark violet colored colonies. The lactose -positive coliforms, such as Enterobacter and Klebsiella grow with large, slimy acting colonies appear pink with a dark center in transmitted light. Very easy to distinguish them is Escherichia coli, which also belongs to the lactose -positive coliforms, on the EMB agar but at the greenish metallic luster of the colonies can be clearly seen, continue the colonies have a blue- black center in transmitted light.
Typical Composition
The soil mostly consists of ( in grams per liter):
- Peptone 10.0
- Dipotassium hydrogen phosphate, 2.0
- Lactose 5.0
- Sucrose 5.0
- Eosin ( yellowish) 0.4
- Methylene blue 0.07
- Agar- agar 13.5