What is Hemolysis

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.

  Subscribe in a reader
Types of hemolysis 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 generally caused by peroxides produced by the bacterium.

The bacterium Staphylococcus aureus, a common cause of infection in humans, secretes a toxin known as alpha-hemolysin that kills human cells by forming holes in their membranes, through which chemicals essential to survival leak out. The movie illustrates the structure of the hole, which is created when identical proteins released by the bacterium (in pink, yellow, gold, red, grey, green, and white) assemble in groups of seven in the cell's membrane (in cyan).

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.

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.