This image illustrates growth of a nonlactose fermenter on MacConkey agar. MacConkey agar contains bile salts and crystal violet which inhibit the growth of gram-positive bacteria. The agar also contains lactose and a red dye that differentiates the lactose fermenters from the nonfermenters. Colonies of lactose fermenting bacteria will be pink to red while the nonfermenters will appear colorless or transparent. However, this agar does not distinguish between the nonlactose fermenters; this growth could indicate several organisms - Proteus, Salmonella or Shigella, for example. In a stool specimen, it would be enough evidence to continue with further identification. This image is useful when discussing metabolism and the concept of differentiating bacteria from one another by the presence of enzymes.

　　Age of culture is 24 h.