The main difference between blood agar and chocolate agar is that blood agar contains whole sheep blood and appears in red color, whereas chocolate agar contains red blood cells lysed by heating and appears in brown to brownish red color.
Blood agar and chocolate agar are two types of agar that contain red blood cells and agar. Generally, they are enriched media that are important for the nonselective growth of fastidious organisms such as pathogenic bacteria.
Key Areas Covered
1. What is Blood Agar
– Definition, Features, Importance
2. What is Chocolate Agar
– Definition, Features, Importance
3. Similarities Between Blood Agar and Chocolate Agar
– Outline of Common Features
4. Difference Between Blood Agar and Chocolate Agar
– Comparison of Key Differences
Key Terms
Blood Agar, Chocolate Agar
What is Blood Agar
Blood agar is a nonselective medium for the growth of pathogenic bacteria like Streptococci. It also serves as a differential medium that differentiates hemolytic bacteria. Generally, it allows hemolysis in the medium. Hemolytic bacteria normally produce cytolytic toxins. These toxins have the ability to lyse red blood cells in the blood agar. This releases the red component of the cells to the medium. Hemolysis makes the medium dark or discolored. In fact, there are three types of hemolysis: alpha hemolysis, which makes the medium dark or discolored, beta hemolysis, which makes the medium clear, and gamma hemolysis which does not change the color of the medium.
Figure 1: Hemolysis
Furthermore, blood agar is a red color medium that is opaque. Blood agar is an enriched medium that contains an array of uncommonly rich nutrients. Since the medium is complex, it is not possible to chemically recreate it. The main uses of blood agar include preparing Salmonella typhi antigens, determining the salinity range of marine Flavobacteria, and determining hemolysis.
What is Chocolate Agar
Chocolate agar is another nonselective, enriched medium important for the isolation of pathogenic bacteria. The main feature of chocolate agar is that it contains red blood cells lysed by heating. Chocolate agar is important for the growth of fastidious bacteria such as Haemophilus influenzae and Neisseria meningitidis. Generally, the growth of these bacteria requires two growth factors that occur inside the red blood cells. They include factor V and factor X. Factor V is nicotinamide adenine dinucleotide, while factor X is hemin. Therefore, red blood cell lysate is the main requirement for the growth of these bacteria.
Figure 2: Neisseria gonorrhoeae Chocolate Agar Plate
Moreover, there are variants of chocolate agar, like bacitracin and Thayer-Martin agar. In general, bacitracin is selective for the genus Haemophilus while Thayer-Martin agar is selective for Neisseria species. The colonies of the Haemophilus influenzae are small, colorless, and moist colonies, and they have a characteristic seminal odor. Neisseria gonorrhea, on the other hand, colonies are small, gray to white colonies that are mucoid with a smooth consistency and defined margins. However, the colonies of Neisseria meningitidis are larger, bluish-grey, round, convex, mucoid, smooth, moist, and glistening colonies.
Similarities Between Blood Agar and Chocolate Agar
- Blood agar and chocolate agar are two types of enriched media that contain blood.
- They are important for the growth of Gram-negative fastidious organisms such as Neisseria and Haemophilus species and Gram-positive cocci.
- They are non-selective media.
- Both are important to indicate hemolysis.
Difference Between Blood Agar and Chocolate Agar
Definition
Blood agar refers to a general-purpose, enriched media to grow fastidious organisms and differentiate bacteria based on their hemolytic properties, while chocolate agar refers to a nonselective, enriched growth medium used for the isolation of pathogenic bacteria.
Red Blood Cells
Blood agar contains red blood cells of whole sheep blood, while chocolate agar contains red blood cells lysed by heating.
Color
Blood agar is red in color, while chocolate agar is brown to brownish red in color.
Cooling Temperature
Sterile sheep blood is added at 50°C to nutrient agar in the preparation of blood agar, while sterile sheep blood is added at 75-80°C to nutrient agar in the preparation of chocolate agar.
Factor V and X
Factor V and X occur inside the red blood cells in blood agar, while Factor V and X occur in the medium in chocolate agar.
Type of Organisms
Blood agar is important for the growth of Neisseria and Haemophilus species, while chocolate agar is important for the growth of S. pyogens and other streptococcus species.
Conclusion
In brief, blood agar and chocolate agar are two types of nonselective media important for the growth of pathogenic bacteria. Generally, blood agar contains red blood cells of whole sheep blood, and it is red in color. It is also important for the growth of Neisseria and Haemophilus species. In comparison, chocolate agar contains red blood cells lysed by heat. Due to the lysis, factor V occurs outside the red blood cells in chocolate agar. In addition, it is important for the growth of S. pyogens and other streptococcus species. Therefore, the main difference between blood agar and chocolate agar is the lysis of red blood cells.
References:
- Blood agar – composition, preparation, uses (VS Chocolate Agar). Laboratoryinfo.com. (2022, March 7). Retrieved January 25, 2023
- Aryal, S. (2023, January 13). Chocolate agar- composition, principle, preparation, results, uses. Microbe Notes. Retrieved January 25, 2023
Image Courtesy:
- “Streptococcal hemolysis” By Y tambe – Own work (CC BY-SA 3.0) via Commons Wikimedia
- “Neisseria gonorrhoeae 01” By w:en:User:MarcoTolo – Own Work (Public Domain) via Commons Wikimedia
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