EMB Agar was first developed by Holt-Harris and Teague. This formula contains both lactose and sucrose with the two indicator dyes, eosin Y and methylene blue. Escherichia and Enterobacter species can be easily distinguished from other Gram negative enteric bacilli using this medium. EMB Agar contains two mildly selective dyes, eosin Y and methylene blue, which interact under strong acidic conditions to impart metallic sheen to Escherichia coli colonies. In addition, E coli produces dark blue-black colonies. Other coliforms produce mucoid, pinkish brown colonies. Non-lactose fermenting bacteria Salmonella and Shigella, produce translucent or colorless colonies. Acinetobacter, Aeromonas, Pseudomonas, Plesiomonas and Yersinia also grow on this medium. Gram-positive bacteria are inhibited by eosin Y. The slow lactose fermenters also ferment sucrose, which reduces the incidence of false-positive interpretations.
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