MacConkey agar

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.

MacConkey agar is a selective and differential culture medium for bacteria. It is designed to selectively isolate gram-negative and enteric (normally found in the intestinal tract) bacteria and differentiate them based on lactose fermentation.[1] Lactose fermenters turn red or pink on MacConkey agar, and nonfermenters do not change color. The media inhibits growth of gram-positive organisms with crystal violet and bile salts, allowing for the selection and isolation of gram-negative bacteria. The media detects lactose fermentation by enteric bacteria with the pH indicator neutral red.[2]

  1. ^ "MacConkey Agar". Texas Medical Center. Archived from the original on 2008-11-04.
  2. ^ Anderson, Cindy (2013). Great Adventures in the Microbiology Laboratory (7th ed.). Pearson. pp. 175–176. ISBN 978-1-269-39068-2.