3.10.1: Blood Agar Test

OER - BIOL 2420 Laboratory Manual · LibreTexts

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    Blood agar is used to grow fastidious bacteria and to differentiate bacteria based on their blood lysis capabilities. Fastidious bacteria are bacteria that require a significant number of growth factors (vitamins, essential amino acids, etc.). Blood is an excellent source of nutrients to meet the needs of bacteria that have complex nutritional requirements; thus, this type of media can be utilized to grow fastidious bacteria.

    Some bacteria are capable of producing enzymes known as hemolysins. Hemolysins cause the lysis of blood cells and may also cause the destruction of hemoglobin. There are three types of hemolysis that may be differentiated using blood agar: alpha (α) hemolysis, beta (β) hemolysis, and gamma (γ) hemolysis.

    Alpha-hemolysis is a partial destruction of the red blood cell. Bacteria capable of alpha hemolysis, such as Streptococcus pneumoniae, do not lyse the red blood cells but rather produce hydrogen peroxide that interacts with heme producing a green discoloration of the media around the colonies. The cell membranes of the red blood cells remain intact, therefore alpha-hemolysis is not a “true” lysis of the blood cell.

    Beta-hemolysis is the result of the action of hemolysins. Streptococcus pyogenes species are among the group of bacteria that can produce these hemolysins. There are two different hemolysins that can be produced; Streptolysin O, which is oxygen sensitive and Streptolysin S, which is stable in the presence of oxygen. These streptolysins function to completely lyse the red blood cell. This type of hemolysis leaves a clear area around the bacterial colonies and is a true lysis of the red blood cells.

    Bacteria that are said to be gamma-hemolytic do not lyse red blood cells at all. Thus, the term gamma-hemolysis can be quite confusing for some. Gamma-hemolytic bacteria grow on the media but the media remains unchanged around the bacterial colonies. Enterococcus faecalis is an example of a bacterium that is gamma-hemolytic.

    Blood Hemolysis Test Results
    Alpha Hemolysis Beta Hemolysis Gamma Hemolysis
    Examples:

    Streptococcus pneumoniae

    Streptococcus mutans    		 Examples:

    Streptococcus pyogenes

    Streptococcus agalactiae    		 Examples:

    Enterococcus faecalis

    Enterococcus faecium
    Blood agar showing alpha, beta, and gamma hemolysis patterns.
    Blood hemolysis types shown on a blood agar. Bacterial samples were inoculated in patterns of the Greek letters representing the type of hemolysis carried out by that bacterial species. Source: Y tambe, Wikimedia Commons, CC BY 2.0.
    Alpha hemolysis is observed where S. pyogenes is seen growing on the blood agar.
    “Beta hemolysis is observed where Streptococcus pyogenes is seen growing on the blood agar.” Source: Nathan Reading, Wikimedia Commons, CC BY 2.0.
    S. agalactiae is observed to demonstrate beta hemolysis on the blood agar plate.
    “Streptococcus agalactiae is observed to demonstrate beta hemolysis on the blood agar plate.” Source: 43trevenque, Wikimedia Commons, CC BY 4.0.

     

    This page titled 3.10.1: Blood Agar Test was last modified on Thu, 08 Oct 2026 15:33:07 GMT and is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by Sherry Smith.