J621/LIFS 675
Medical Microbiology
Mary T. Johnson, Ph.D.
Hemolysis on Blood Agar
Hemolysis on blood agar is used for the preliminary or confirmatory identification of many
types of clinically important bacteria. While it is factored into the differential diagnosis of a
specific infectious agent, hemolysis type is not specific enough to be a final diagnosis criterion.
The three hemolysis conditions continue to be described by terms that are somewhat confusing.
- Alpha-hemolysis is a greenish discoloration of the blood agar surrounding a bacterial
colony; it is a characteristic
of
Streptococcus pneumoniae.
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- Beta-hemolysis indicates a zone of clearing in the blood agar in the area surrounding
a bacterial colony. It is a characteristic of
Streptococcus pyogenes
as well as some strains of
Staphylococcus aureus.
- Gamma-hemolysis is actually a lack of hemolysis in the area surrounding a bacterial
colony growing on blood agar. In fact, culture of bacteria on blood agar for the
purpose of hemolysis classification is performed at 37oC in the presence of 5%
CO2. This results in an overall brownish
discoloration of the blood agar, from its original blood-red hue. An uninoculated blood agar plate (BAP) is shown on the left, above.
Gamma-hemolysis
would therefore describe bacterial growth that results in neither a greenish tinge to the
discoloration (alpha-hemolysis) nor a clear zone that the observer "could read
a newspaper through" (beta-hemolysis).
E-mail Dr. Johnson
for comments or suggestions.
©Copyright 1996-2001, Indiana University School of Medicine
Last modified July 14, 2001