What is mcConkey agar?

MacConkey agar is a selective medium designed to grow Gram negavite bacteria and stain them for lactose fermentation. It contains bile salts[to inhibit most Gram negative bacteria and some species of Staphlococcus], crystal violet dye, neutral red dye ,lactose and peptone. It was first developed by Alfred Theodore MacConkey. It is used to distinguish between lactose fermenting bacteria and non lactose fermenting bacteria. By utilising the lactose available in the medium, Lac+ bacteria such as Escherichia coli, Enterobacter and Klebsiella will produce acid, which lowers the pH of the agar below 6.8 and results in the appearance of red/pink colonies. Non-Lactose fermenting bacteria such as Salmonella, Proteus species and Shigella cannot utilize lactose, and will use peptone instead. This forms ammonia, which raises the pH of the agar, and leads to the formation of white/colorless colonies