for isolation, enumeration and differentiation of members of Enterobacteriaceae.
EMB AGAR or EOSIN METHYLENE BLUE AGAR, LEVINE (VEG.) is used for isolation, enumeration and differentiation of Enterobacteriaceae. First EMB was introduced by Holt - Harris and Teague in 1916 to differentiate Escherichia spp. and Aerobacterspp. Later, modified by Levine in 1918 who has increased the Lactose content by removing Sucrose from the formula. The medium contains Veg. Peptone source of carbon and nitrogen for the growth of microorganisms. Lactose is the fermentable carbohydrates. Differentiation of enteric bacteria is possible due to the presence of the sugars lactose 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. Dipotassium phosphate is the buffer. Eosin Y and Methylene blue are dyes that combine to form a complex at an acid pH. 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. At a sufficiently low pH, strong lactose fermenters such as Escherichia coli produce colonies with a green metallic sheen. This metallic green sheen is an indicator of vigorous lactose 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 non-lactose fermenters appear as translucent or pink. Agar is the solidifying agent.