VetBact

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.

Other comments:

• Lactose fermenting bacteria (Escherichia coli, Enterobacter spp. and Klebsiella spp.) form colonies that seem to be red or pink, because acid is formed and the medium change colour 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.
• Some gram negative bacteria such as the bacteria belonging to Family Pasteurellaceae do not grow on MacConkey agar.

Updated: 2021-10-10.