EMB AGAR

Eosin Methylene Blue (EMB) Agar was originally devised by Holt-Harris and Teague and further modified by Levine. The medium is a combination of the Levine and Holt-Harris and Teague formulae which contains peptone and phosphate as recommended by Levine and two carbohydrates as suggested by Holt-Harris and Teague. This makes EMB Agar suitable for selective isolation. These dyes serve as differential indicators in response to the fermentation of carbohydrates. The ratio of eosin and methylene blue is adjusted approximately to 6:1. Sucrose is added to the medium as an alternative carbohydrate source for typically lactose-fermenting, gram-negative bacilli, which on occasion do not ferment lactose or do so slowly.

The coliforms produce purplish black colonies due to taking up of methylene blue-eosin dye complex, when the pH drops. Such colony appearance is commonly observed on EMB Agar. Non fermenters probably raise the pH of the surrounding medium by oxidative deamination of protein, which solubilizes the methylene blueeosin complex resulting in colourless colonies. Some strains of Salmonella and Shigella species do not grow in the presence of eosin and methylene blue. Further tests are required to confirm the isolates. The medium consists of Peptone which serves as a source of carbon, nitrogen, and other essential growth nutrients. Lactose and sucrose are the sources of energy by being fermentable carbohydrates. Eosin-Y and methylene blue serve as differential indicators. Phosphate buffers the medium.