Re: Why do they use sheeps blood in agar?

Hi John -

Sheep's blood is added to a nutritional agar base for a few reasons:

1. Added nutrients, present in the blood
2. To measure hemolysis. Hemolytic organisms can break down the red cells, or "lyse" them, to release the nutrients within. These nutrients include the iron bound in hemoglobin, a necessary element for bacterial growth, and other vitamins and nutrients present within the red cell. There are three types of hemolysis"
   • Alpha hemolysis: incomplete hemolysis, produces green zone around the bacterial colon. This pattern is very characteristic for disease-causing bacteria such as Streptococcus pneumoniae, a cause of pneumonia.
   • Beta hemolysis: complete hemolysis that produces a clear zone around the bacterial colony, as the red cells are completely broken down. Bacteria such as Staphylococcus aureus and Escherichia coli produce this pattern.
   • Gamma hemolysis, "or no hemolysis present" - organisms that are unable to break down red cells. Examples include Staphylococcus epidermidis and some species of enterococci.

-Lynn

Ref: Diagnostic Microbiology, by Koneman.