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MacConkey agar

MacConkey agar

The image shows three plates with MacConkey agar. No bacteria were cultivated on the plate in image A. Escherichia coli and Proteus vulgaris have been cultivated during incubation 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.

Image: Karl-Erik Johansson (BVF, SLU) and Lise-Lotte Fernström (BVF, SLU)

Applications

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:

  • peptone  (enzymatic digest of animal tissues or milk, which constitute a source of nutrients)
  • lactose (to differentiate between lactose fermenters and non-fermenters)
  • bile salts (inhibits gram positive bacteria)
  • crystal violet (inhibits gram positve bacteria)
  • NaCl (provides the osmotic balance)
  • neutral red (pH indicator)
  • agar (solidifying agent)
  • H2O

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.

Summary:

  • Lactose fermenting bacteria (Escherichia coli, Enterobacter spp. and Klebsiella spp.) form red or pink colonies because acid is formed when lactose is fermented. 
  • Non-lactose fermenting bacteria (Proteus spp,  Pseudomonas aeruginosa, Salmonella spp and Shigella spp.) form yellowish, white or colourless and translucent colonies.

Updated: 2017-03-31.


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Swedish University of Agricultural Sciences