| Some bacteria produce hemolysins, exotoxins that cause
red blood cells (RBC's) to burst open (hemolyse). When these bacteria
are cultured on blood agar, this hemolysis is visible as an area of clearing
around the colony (zone of hemolysis). If the organism produces enzymes
that completely lyse the RBC's, this is termed beta hemolysis. Partial
destruction of the RBC's produces a greenish color to the zone of hemolysis
and is termed alpha hemolysis. Organisms lacking hemolysins cause
no change in the color or opacity of the media and are termed gamma
hemolytic.
Production of hemolysins is associated with virulence.
Streptococcus pneumoniae,  the major cause of community-acquired
bacterial pneumonia is alpha-hemolytic, while Streptococcus pyogenes (Group A Strep), the cause of "Strep throat", is beta-hemolytic. Staphylococcus
aureus, which causes toxic shock syndrome, impetigo, food poisoning
and other diseases is usually beta-hemolytic.
Click on the image to see an enlarged version.
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1. blood agar
3 types of hemolysis |
2. blood agar
gamma hemolysis |
3. blood agar
alpha hemolysis |
4. blood agar
beta hemolysis |
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