Microbiology - Cell Cultures and Characteristics - Discussion

It differentiates the gram-positive from gram-negative due to the fact that it has crystal violet and lactose fermentors from non-lactose fermentors by having the lactose as one of the nutrients in it.

Certain media are designed in such a way that different bacteria can be recognized on the basis of their colony colour. Various approaches include incorporation of dyes, metabolic substrates etc so that those bacteria that utilize them appear as differently coloured colonies. Such media are called differential media or indicator media. Differential media allow the growth of more than one microorganism of interest but with morphologically distinguishable colonies.

Examples; Mac-Conkey medium.