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The image shows three plates with MacConkey agar. No bacteria were streaked onto the plate in image A. Escherichia coli and Proteus vulgaris have been streaked and incubated at 37°C on the plates in the images B and C. Note the colour of the colonies, the, precipitation zone on the plate in image B and that P. vulgaris does not swarm on the plate in image C, which does not contain NaCl. - Click on the image to enlarge it.
MacConkey agar is a selective culture medium, which is used to grow gram negative bacteria and to identify those which ferment lactose. It has proved useful for identification of intestinal bacteria. Purple agar (with lactose) is used in some laboratories instead of MacConkey agar
The medium contains:
The final pH of the medium should be 7.1. Many variants of MacConkey agar exist and e.g. NaCl can be omitted if you don't want Proteus spp. to swarm. Bile salts and crystal violet inhibit most gram positive bacteria. Neutral red is colorless at any pH greater than 6.8 and turns red at a pH below 6.8.