Eosin Methylene Blue (EMB) Agar was originally devised by Holt-Harris and Teague (1) and further modified by Levine (2).
The above medium is a combination of the Levine and Holt-Harris and Teague formulae which contains peptic digest of animal
tissue and phosphate as recommended by Levine and two carbohydrates as suggested by Holt-Harris and Teague.
Methylene blue and Eosin-Y inhibit gram-positive bacteria to a limited degree. 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. The dye complex is absorbed into the colony. Nonfermenters probably raise the
pH of surrounding medium by oxidative deamination of protein, which solubilizes the methylene blue-eosin complex resulting
in colourless colonies (3). 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.
Peptic digest of animal tissue serves as 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.
The test sample can be directly streaked on the medium plates. Inoculated plates should be incubated, protected from light.
However standard procedures should be followed to obtain isolated colonies. A non-selective medium should be inoculated in
conjunction with EMB Agar. Confirmatory tests should be further carried out for identification of isolated colonies.