MacConkey (also McConkey) agar is a culture medium designed to grow Gram-negative bacteria and stain them for lactose fermentation. It contains bile salts, crystal violet dye (to inhibit Gram-positive bacteria), neutral red dye (which stains microbes fermenting lactose), lactose and peptone. Alfred Theodore MacConkey developed it while working as a bacteriologist for the Royal Commission on Sewage Disposal.
Acting as a visual 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 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 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.
Thus, by selecting for Gram-negative bacteria and differentiating between enteric pathogens such as E. coli and Salmonella, the MacConkey agar is readily implemented in the clinical diagnosis of diarrhea.