Blood Agar

As the name suggests, this is media prepared using blood as a primary component. For the classroom this is often sheep's blood, though for some clinical applications human blood is required for a good result. These tests are focused on the effects of hemolysins, proteins that can break down blood. The agar starts out opaque and blood red (obviously). When organisms grow on the media they cause one of three reactions.

Alpha hemolysis: Partial digestion of red blood cells, causes a dark discoloration around the colonies.

Beta hemolysis: Complete digestion of red blood cells, causes clearing around the colonies that lose color and become completely transparent.

Gamma hemolysis: Inert reaction, no clearing or change, the organism does not react with red blood cells.