Blood Agar- Composition, Principle, Preparation, Uses and Hemolysis


Blood agar is a type of bacterial growth medium that contains blood as an additional source of nutrients. Blood agar is used to culture fastidious organisms, which are bacteria that have specific nutritional requirements and do not grow well on ordinary media. Blood agar is also used to differentiate bacteria based on their ability to hemolyze (break down) red blood cells in the blood.

Blood agar consists of a base medium, such as tryptic soy agar or Columbia agar, that provides the basic nutrients for bacterial growth, such as proteins, carbohydrates, salts, and vitamins. To this base medium, 5% of defibrinated mammalian blood (usually sheep or horse) is added after sterilization. The blood provides extra nutrients, such as iron, growth factors, and hemin, that are essential for some bacteria. The blood also serves as an indicator of hemolysis, which is a characteristic reaction of some bacteria that produce enzymes called hemolysins. Hemolysins can cause partial or complete lysis of red blood cells, resulting in different patterns of color change on the agar surface.

There are four types of hemolysis that can be observed on blood agar: alpha (α), beta (β), gamma (γ), and alpha prime (α`). Alpha hemolysis is a partial lysis of red blood cells that causes a greenish discoloration around the bacterial colonies. Beta hemolysis is a complete lysis of red blood cells that causes a clear zone around the bacterial colonies. Gamma hemolysis is no lysis of red blood cells and no change in color on the agar. Alpha prime hemolysis is a double zone of hemolysis, with a narrow zone of intact red blood cells surrounded by a wider zone of complete lysis.

The type of hemolysis produced by a bacterium can be used as a clue for its identification. For example, Streptococcus pneumoniae produces alpha hemolysis, Streptococcus pyogenes produces beta hemolysis, and Staphylococcus epidermidis produces gamma hemolysis. However, hemolysis alone is not sufficient for identification, as different bacteria can produce the same type of hemolysis or different types of hemolysis depending on the source of blood used. Therefore, other tests and characteristics are also needed to confirm the identity of a bacterium.

Blood agar is an important clinical medium that can help isolate and identify fastidious and pathogenic bacteria from various specimens. Blood agar can also be modified by adding other ingredients or heating the blood to create specialized media for specific purposes. For example, chocolate agar is a heated blood agar that supports the growth of Neisseria and Haemophilus species. Phenolphthalein phosphate can be added to blood agar to detect phosphate-producing Staphylococci.

In this article, we will discuss the composition, principle, preparation, uses and limitations of blood agar in detail. We will also explain how to interpret the results on blood agar based on the growth and hemolysis patterns of different bacteria.