A differential medium is one that permits growth of some microorganisms but not others. There are many examples of this. Acid may be added to a medium to allow for the growth of fungi and to prevent the growth of bacteria. Potato Dextrose Agar is a medium used for this purpose. Bile may be added to a media to encourage the growth of one bacterium over all others.
Violet Red Bile Agar is an example of this.
Note: bacterium is singular; the word, bacteria, is plural.