MacConkey Broth is used as a presumptive test medium for the presence of coliforms in water and other materials of sanitary importance. It is also used for cultivating Gram negative, lactose-fermenting bacilli in water and foods. The formation of gas and acid confirms the presence of coliforms, as demonstrated by the change of the medium color from purple to yellow. Pancreatic digest of gelatin provides nitrogen, vitamins, minerals and amino acids essential for growth. Lactose is a fermentable carbohydrate causing a drop in the pH and subsequently a color change of the pH indicator (Bromocresol purple) and bile precipitation. Ox bile is a selective agent to inhibit the growth of Gram positive organisms. The European Pharmacopoeia, USP recommends this media in the paragraph 2.6.13: "Microbiological examination of non-sterile products: Test for specified microorganisms" for the growth promoting and inhibitory properties of the media in the test for E.coli. Also, this medium is recommended for the testing of E.coli in products.
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