Blood agar is a type of growth medium used in laboratories to culture bacteria. It is made by adding sheep or horse blood to a base of nutrient agar, which provides essential nutrients for bacterial growth. The blood in the agar allows for the observation of hemolysis, a process where bacteria break down red blood cells. This medium is particularly useful for identifying different types of bacteria based on their ability to lyse red blood cells. For example, some bacteria produce enzymes that cause complete lysis, resulting in a clear zone around their colonies, while others may only partially lyse the cells, creating a greenish discoloration.