Mechanism of the Selective Action of Eosin-Methylene-Blue Agar on the Enteric Group

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Abstract
The actual mechanism of the differentiation of lactose-fermenting and non-lactose-fermenting organisms on eosin-methylene-blue medium is not reported in the literature. The present study is an attempt to elucidate this problem. The color of colon forms on E.M.B. agar was found to depend on 2 factors: (1) the reaction of eosin with methylene blue to form a dye compound of either acidic or neutral nature, and (2) the production, by lactose-fermenting colonies, of a sufficiently low pH so that this dye compound is taken up by individual cells of the colony. Non-lactose-fermenting organisms are not colored because the compound is not taken up in alkaline reaction. An explanation is offered to account for the occasional blue colonies found on E.M.B. medium. These colonies may form a relatively high pH and thus cause slight dissociation of the compound, thus allowing independent staining of the colonies by methylene blue.