A differential medium enlists a
particular bacterial property to allow visual differentiation of one organism from another
(e.g., ability to ferment a particular carbohydrate like lactose alters a pH indicator and a
lactose-fermenter colony has a distinctive color compared to lactose non-fermenters
which are not colored). Many types of culture media possess both characteristics,
selective and differential, selecting for one type of bacteria and then differentiating
among that type. There are hundreds of selective and differential media, but a few of the
more common ones as well as those used for general purposes are described below: