MacConkey agar

It is designed to selectively isolate gram-negative and enteric (normally found in the intestinal tract) bacteria and differentiate them based on lactose fermentation.

The media inhibits growth of gram-positive organisms with crystal violet and bile salts, allowing for the selection and isolation of gram-negative bacteria.

[4] The medium was developed by Alfred Theodore MacConkey while working as a bacteriologist for the Royal Commission on Sewage Disposal.

[5] Using neutral red pH indicator, the agar distinguishes those gram-negative bacteria that can ferment the sugar lactose (Lac+) from those that cannot (Lac-).

By utilizing 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 pink colonies.

Plate with an active bacterial culture
With an active bacterial culture
Lactose
The organism on the left is a lactose fermenter , as evidenced by the pink color. The organism on the right produced no color, so it does not appear to be a lactose fermenter.