Hemolyses of Streptococcus spp.
(left) α-hemolysis (S. mitis);
(middle) β-hemolysis (S. pyogenes);
(right) γ-hemolysis (= non-hemolytic, S. salivarius)

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

Hemolysis is the breakdown of red blood cells. The ability of bacterial colonies to induce hemolysis when grown on blood agar is used to classify certain microorganisms. This is a particularly useful in classifying streptococcal species. A substance that causes hemolysis is a hemolysin.

[edit] Types of hemolysis

[edit] Alpha

When Alpha hemolysis (α-hemolysis) is present the agar under the colonies is dark and greenish. Streptococcus pneumoniae and a group of oral streptococci (Streptococcus viridans or viridans streptococci) 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 caused by hydrogen peroxide produced by the bacterium, oxidizing hemoglobin to green methemoglobin.

[edit] Beta

Beta hemolysis (β-hemolysis), sometimes called complete hemolysis, is a complete lysis of red cells in the media around and under the colonies: the area appears lightened and transparent. Streptococcus pyogenes, or Group A beta-hemolytic Strep (GAS), displays beta hemolysis.

Some weakly beta-hemolytic species 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.

[edit] Gamma

If an organism does not induce 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.

[edit] Notes

¹The CAMP test is so called from the initials of those who initially described it, R. Christie, N. E. Atkins, and E. Munch-Peterson. It allows to classify Streptococcus agalactiae from the others.

[edit] References

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