EMB agar and MacConkey agar are differential and selective media. The selective components in EMB agar are eosin and methylene blue dyes, whereas crystal violet dye and bile salts are the selective components in MacConkey agar. Lactose is the differential component in both media.
The selective components in EMB agar and MacConkey agar inhibit growth of Gram-positive microorganisms but allow the growth of Gram-negative species. The microogranisms are differentiated based on their ability to ferment lactose present in the medium for energy.
Lactose-fermenting microogranisms produce black and sometimes shiny colonies on EMB agar, whereas nonlactose fermenters produce light-pink or uncolored colonies. Production of acid from the fermentation of lactose leads to precipitation of the dyes in EMB agar on the colony surface, leading to specific color changes.
When using MacConkey agar, lactose-fermenting microorganisms produce red- to pink-colored colonies, whereas the nonlactose fermenters produce transparent or colorless colonies. Lactose fermenters produce an acid in the medium that lowers the pH of the agar, leading to the color changes. Nonlactose fermenters use peptone instead of lactose, producing ammonia. This raises the pH of the MacConkey agar, leading to the formation of colorless or white colonies.
EMB agar and MacConkey agar can be used for the isolation and characterization of coliforms from urine, water, stool and other material.