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Examples of alpha (top), beta (middle), and gamma (bottom) hemolysis on sheep blood agar plates

Hemolysis is used in the empirical identification of microorganisms based on the ability of bacterial colonies grown on agar plates to break down red blood cells in the culture. When the organism has been grown on blood agar plates, it can be classified with regard to whether or not it has caused hemolysis in the red blood cells (RBCs) incorporated in the medium. This is of particular importance in the classification of streptococcal species. A substance that causes hemolysis is a hemolysin.

While a blood agar plate may contain varying concentrations of RBCs and may use blood from a variety of animals, clinical results are most often reported on 5-10% sheep blood agar plates (SBAP).

• Alpha hemolysis (α-hemolysis) is present if there is a greenish darkening of the agar under the colonies. Streptococcus pneumoniae and Streptococcus viridans display alpha hemolysis. This is sometimes called green hemolysis because of the color change in the agar. Other synonymous terms are incomplete hemolysis and partial hemolysis. Alpha hemolysis is generally caused by peroxides produced by the bacterium.

• Beta hemolysis (β-hemolysis) is caused by a complete lysis of the red cells in the media. The area around and under the colonies are lightened and transparent. Streptococcus pyogenes, or Group A beta-hemolytic Strep, displays beta hemolysis. Hemolytic tests are also used to differentiate Staphylococcus aureus, which displays beta-hemolysis, from S. epidermidis, a non-hemolytic, commensal species. Beta hemolysis is sometimes called complete hemolysis. Beta hemolysis is caused by hemolysins.
  • Some weakly beta hemolytic species will cause intense beta hemolysis when grown together with a strain of Staphylococcus. This is called the CAMP test¹. Streptococcus agalactiae displays this property. Clostridium perfringens can be identified presumptively with this test.

• If an organism does not cause hemolysis, it is said to display gamma hemolysis (γ-hemolysis): the agar under and around the colony is unchanged (this is also called non-hemolytic). Enterococcus faecalis (formerly called Group D Strep) displays gamma hemolysis.

¹The CAMP test is so called from the initials of those who initially described it, R. Christie, N. E. Atkins, and E. Munch-Peterson.

• Ryan KJ; Ray CG (editors) (2004). Sherris Medical Microbiology, 4th ed., McGraw Hill. ISBN 0-8385-8529-9.