Hemolysis refers to the breakdown of erythrocytes, or red blood cells. In microbiology, bacteria may be classified based on their ability to induce hemolysis on a blood agar medium. Three types of hemolysis can occur and are classified as alpha, beta, and gamma hemolysis. Each type is characterized by the differential physical characteristics and varying degrees of lysis of the blood cells due to the specific hemolysins, or chemicals that induce hemolysis, secreted by particular bacteria.
Alpha hemolysis presents with a color change in the initially red agar to a very dark green color. This results from the oxidation of hemoglobin to methemoglobin by hydrogen peroxide secreted by the bacteria. In addition, alpha hemolysis does not result in complete lysis of the blood cells and is therefore often referred to as partial or incomplete hemolysis.
Beta hemolysis refers to the complete lysis of the blood cells. It presents as a transparent, yellow color on the blood agar medium. This type of hemolysis occurs due to an enzyme produced by bacteria called streptolysin. This enzyme interacts with cholesterol in the cellular membrane resulting in deterioration of this protective cellular structure.
Gamma hemolysis is displayed by bacteria that do not induce hemolysis of the blood cells. These organisms are referred to as nonhemolytic and are identifiable based on a lack of change in color or transparency to the medium directly under the bacterial colonies.
Uses of Blood Agar
Blood agar is mainly used to identify and grow various strains of pathogenic, or harmful, bacteria. The strains that display alpha and beta hemolysis have exotoxins that may display harmful effects within the body as evidenced by their breakdown of erythrocytes in the medium. The streptolysin used by beta-hemolytic bacteria, in particular, has the potential to destroy many different types of cells in the body due to its deterioration of the cell membrane.