Blood agar is called blood agar because it contains blood.
Understanding Blood Agar
Blood agar is a commonly used growth medium in microbiology. It is named for its crucial ingredient: blood. The addition of sterile, defibrinated blood to a base of nutrient agar is what transforms a standard medium into blood agar. This addition is not done before autoclaving but after, once the base agar is cooled to between 45–60°C. The specific temperature range is key to avoiding degradation of the blood components during the mixing.
How Blood is Added
The process of adding blood to the agar is as follows:
- A nutrient agar mixture is prepared and sterilized by autoclaving.
- After autoclaving, the mixture is cooled to 45–60°C.
- Sterile defibrinated blood is added to the cooled agar base.
- The blood and agar are mixed thoroughly.
- The resulting blood agar is immediately poured into petri dishes to solidify.
Why Use Blood?
The blood added to the agar serves several important functions:
- Enrichment: Blood provides essential growth factors for many fastidious microorganisms.
- Differentiation: The interaction between microbial enzymes and the blood allows for the differentiation of organisms based on their hemolytic patterns (breakdown of red blood cells).
Types of Hemolysis on Blood Agar
Observing the changes in the blood around a colony can help in the identification of bacteria, such as:
- Alpha Hemolysis (α): Partial breakdown of red blood cells, creating a greenish or brownish zone around the colony.
- Beta Hemolysis (β): Complete breakdown of red blood cells, creating a clear zone around the colony.
- Gamma Hemolysis (γ): No breakdown of red blood cells, with no change in the appearance of the agar around the colony.
Blood Agar vs. Chocolate Agar
While both blood agar and chocolate agar contain blood, they differ in preparation. In chocolate agar, the blood is added to the medium before it's cooled. The heat lyses (breaks open) the red blood cells, releasing their contents and turning the medium chocolate brown.
| Feature | Blood Agar | Chocolate Agar |
|---|---|---|
| Blood Added | After cooling, at 45-60°C | Before cooling, while still hot |
| Blood Cells | Intact | Lysed |
| Color | Red | Chocolate brown |
| Use | Hemolysis observation | Growth of fastidious organisms |
The term "blood agar" directly reflects its key component, which is sterile, defibrinated blood, making it straightforward and descriptive. The process of mixing the blood after cooling allows for the observation of hemolysis patterns, a critical diagnostic tool in microbiology.
