EMB agar contains peptone, lactose, sucrose, and the dyes eosin Y and methylene blue; it is commonly used as both a selective and a differential medium. EMB agar is selective for gram-negative bacteria. The dye methylene blue in the medium inhibits the growth of gram-positive bacteria; small amounts of this dye effectively inhibit the growth of most gram-positive bacteria. Eosin is a dye that responds to changes in pH, going from colorless to black under acidic conditions. EMB agar medium contains lactose and sucrose, but not glucose, as energy sources. The sugars found in the medium are fermentable substrates which encourage growth of some gram-negative bacteria, especially fecal and nonfecal coliforms. Differentiation of enteric bacteria is possible due to the presence of the sugars lactose and sucrose in the EMB agar and the ability of certain bacteria to ferment lactose in the medium. Lactose-fermenting gram-negative bacteria (generally enteric) acidify the medium, and under acidic conditions the dyes produce a dark purple complex which is usually associated with a green metallic sheen. This metallic green sheen is an indicator of vigorous lactose and/or sucrose fermentation ability typical of fecal coliforms. A smaller amount of acid production, which is a result of slow fermentation (by slow lactose-fermenting organisms), gives a brown-pink coloration of growth. Colonies of nonlactose fermenters appear as translucent or pink
If you are maintaining pH at 7.2 means you will get good metallic sheen colonies.
@bharathy: thnak you for the info. I have maintained the optimal PH (maintained it in range of 7.1 - 7.4) yet no sheen observed. do you know how can we enhance the sheen production by E.coli?
- Badges
- Science topic