Blood agar

Blood agar is a type of growth medium used in microbiology to cultivate bacteria and to detect and differentiate hemolysis. It is a nutrient-rich medium that contains sheep blood.

Composition

Blood agar is composed of tryptic soy agar or heart infusion agar, and 5% sheep blood. The blood is added after the autoclaving process to preserve the factors that facilitate the growth of fastidious organisms and the detection of hemolysis.

Uses

Blood agar is used to isolate and cultivate many types of bacteria, including Streptococcus and Staphylococcus species. It is also used to differentiate bacteria based on their hemolytic properties.

Hemolysis

Hemolysis is the breakdown of red blood cells. On blood agar, bacteria can be classified into three types based on their hemolytic properties:

• Alpha hemolysis (α-hemolysis): This is a partial or green hemolysis associated with a reduction in red blood cell hemoglobin. Alpha-hemolytic bacteria produce a greenish discoloration on the agar.
• Beta hemolysis (β-hemolysis): This is a complete or clear hemolysis. Beta-hemolytic bacteria produce a clear zone around the colonies.
• Gamma hemolysis (γ-hemolysis): This is a non-hemolytic reaction. Gamma-hemolytic bacteria do not cause any change in the blood agar.

Limitations

While blood agar is a versatile growth medium, it has limitations. Some bacteria, such as Haemophilus influenzae, require factors found in blood but do not grow well on blood agar. Other bacteria, such as Mycobacterium tuberculosis, do not grow on blood agar at all.

See also

• Chocolate agar
• MacConkey agar
• Sabouraud agar

Blood agar Resources
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