Microbiology uses different types of media in order to distinguish the microorganisms from one another. Different microorganisms need different conditions to be able to grow, hence, using different media that have specific characteristics that allow one type of microorganism to grow is the key in identification process.
For example, if the type of medium is nutritive, the identification process becomes difficult since the nutritive medium allows the growth of many types of microorganisms. The identification process becomes less difficult if the type of medium is differential.
There exists one type of medium that allows the growth of some groups of microorganisms, but it inhibits the development of other groups. These types of media are called selective media. Some examples of selective media are PEA (Phenylethyl Alcohol) agar, mannitol salt agar, and EMB (Eosine Methylene Blue) agar. The last medium (EMB) is also a differential medium and it can be considered the inhibitor of gram-positive organisms. Unlike EMB, PEA is considered an inhibitor of gram-negative organisms.