Selective/Differential Media

Selective Media:
Media that prevents some bacteria's growth

ex: Mannitol Salt Agar: 75% NaCl— most bacteria can't grow in high salt
A bacteria that can grow in high salt is S. auareus can grow in high salt.
Since MSA contains mannitol, if bacteria ferment mannitol, agar changes from pink to yellow.

ex: Eosin-Methylene Blue (EMB)
Prevents growth of gram-positives, and also contains lactose.
If bacteria on the plate can ferment lactose, bacteria on plate will appear metallic green

Differential Media:
Media that lets you see whether bacteria can eat different things

ex: DNase agar: if bacteria can break down DNA, agar changes from blue-green
to clear where bacteria are.