Blood agar showing Alpha-hemolysis, which indicates normal flora
Blood agar showing Alpha-hemolysis, which indicates normal floraBlood agar showing Alpha-hemolysis, which indicates normal floraBlood agar showing Alpha-hemolysis, which indicates normal flora
Blood agar showing Alpha-hemolysis, which indicates normal flora
Blood Agar Bacterial Growth Medium Showing Beta Hemolysis
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Blood Agar (BAP) Bacterial Growth Medium - P2
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Certain bacteria produce exotoxins called hemolysins, which act on the red blood cells to lyse, or break them down.


Hemolysis Patterns of Blood Agar
Beta hemolysis means that the bacteria's hemolytic exotoxins completely beak down the blood cells. The β-hemolysis pattern results in the media displaying clear halos around bacterial colonies.

Page last update: 3/2016


When Is Blood Agar (BAP) Used?
Blood agar is usually inoculated from a patient’s throat swab, because the medical laboratory is trying to detect the presence of Group A beta hemolytic Streptococci (a Gram-positive round shaped bacteria that causes beta-hemolysis on blood agar.) The major human pathogen in this group is Streptococcus pyogenes, the causative agent of strep throat. Normal throat flora will exhibit alpha or gamma hemolysis.

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Sources and Helpful Link
  • Schauer Cynthia (2007) Lab Manual to Microbiology for the Health Sciences, Kalamazoo Valley Community College.
  • Bauman, R. (2014) Microbiology with Diseases by Taxonomy, Pearson Benjamin Cummings.

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Blood agar (BAP) is a differential growth medium which microbiologists use to distinguish clinically significant bacteria from throat and sputum cultures. BAP contains 5% sheep blood.

Blood Agar Is a Differential Medium
A growth medium is considered differential if, when specific microbes are present, the medium or bacterial colonies themselves exhibit a color change that provides information about their identity.
Alpha hemolysis (α-hemolysis) means that the bacteria generate chemicals that only partially break down the blood cells. This results in the media showing a brownish discoloration (like a bruise) around the colony, indicating incomplete hemolysis.

Blood agar showing Alpha-hemolysis, which indicates normal flora
Alpha Hemolysis
Throat swab that resulted in Beta-hemolysis, indicating the presence of pathogenic bacteria Steptococcus pyogenes, the cause of 
strep throat. 
Gamma hemolysis is essentially no hemolysis at all. The bacteria have no effect on the red blood cells, and there is no change to the color of the medium.

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