How to Interpret Beta Hemolysis on Blood Agar
Test for Strep Throat – Identifying B-hemolytic Streptococcus
Feb 9, 2009
Some growth media, such as Blood Agar (BAP), can be used to do more than just grow bacteria; specialized agars can be used to help identify microbes.
Blood Agar Is a Differential Medium
A growth medium is considered differential if, when specific microbes are present, the medium exhibits a color change due to the metabolic capabilities of the bacteria growing on it. This color change provides information about the bacteria’s identity.
Blood agar (BAP) is a differential growth medium used to distinguish clinically significant bacteria from throat and sputum cultures. In clinical diagnosis, Blood agar is usually inoculated from a patient’s throat swab.
BAP contains 5% sheep blood. Certain bacteria produce enzymes (hemolysins) that act on the red blood cells in BAP to lyse or break them down.
Hemolysis Patterns of Blood Agar
There are three possible hemolysis patterns that can be observed when organisms are growing on Blood Agar (BAP):
- Beta hemolysis means that the bacteria's hemolytic enzymes completely beak down the blood cells. The β-hemolysis pattern results in the media displaying clear halos around bacterial colonies. Streptococcus pyogenes, strep throat bacteria, is the B-hemolytic organism that clinicians are looking for when they plate a throat sample onto BAP.
- Alpha hemolysis (α-hemolysis) means that the bacterial enzymes only partially break down the blood cells. This results in the media showing a yellowish/greenish/brownish discoloration (like a bruise) around the colonies, indicating incomplete hemolysis.
- Gamma hemolysis is essentially no hemolysis at all; that the bacteria have no effect on the red blood cells and there is no change to the color of the medium.
What If a Blood Agar Plate Shows Beta Hemolysis?
Beta hemolysis on BAP does not always indicate strep throat. There are other microbes that will produce B-hemolysis, including some Gram negative enteric bacteria (poop bacteria).
First look at the colony morphology—the shape and surface characteristics of the dots on the agar. These dots are colonies; millions of bacteria that have resulted from the one initial bacterium multiplying. Streptococcus pyogenes forms very tiny (punctiform) colonies, in contrast to Gram-negative bacteria, which typically form larger, slimy-looking colonies.
In addition to examining the colony morphology, there is a more precise way to determine if the B-hemolytic bacteria are Strep. Plating another throat sample onto Bood Agar, then adding a bacitracin antibiotic disk will help reveal if the unknown bacteria are S. pyogenes. Bacitracin inhibits the growth of Streptococcus. Therefore, if, after incubation, the plate shows Beta-hemolytic colonies that will not grow near the bacitracin disk, this is indicative to Strep throat bacteria.
About the Hemolysis Images
The first photo associated with this article shows beta hemolysis. The medium around the bacterial colonies has lost its color and is now clear. The colonies are punctiform. This is the type of colony morphology exhibited by S. pyogenes (strep throat bacteria).
The second photo associated with this article displays bacterial colonies growing on Blood Agar. The medium was inoculated from a throat culture. Around the colonies growing, the agar has turned a brownish-yellow, like a bruise. This color change indicates that alpha-hemolytic normal flora are present.
Additional Microbiology Resources
For more information on Microbiology, see the SPO Virtual Microbiology Classroom, or Todar’s Online Textbook of Bacteriology.
Sources
Schauer, Cynthia (2007) Lab Manual to Microbiology for the Health Sciences, Kalamazoo Valley Community College.
Bauman, R. (2005) Microbiology. Pearson Benjamin Cummings.
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