Hearts, brains, and blood. These are some of the ingredients used to grow bacteria. Since the introduction of agar to bacterial cultivation methods by Fanny Hesse, microbiologists have been creating variations of agar medium to accommodate microbes that have different requirements for growth.
The ingredient up for discussion today: blood.
What is Blood Agar and How Does It Work?
As the name suggests, blood agar contains blood, sourced typically from sheep or horse. The blood is added to a base medium to a final volume of 5 – 10%. And, a fun fact: it’s likely that human blood has been used in the past. (We now know the use of human blood is discourage as it can introduce exposure to human blood-borne pathogens.)
So why blood? The addition of blood to agar provides additional nutrients to help certain bacteria grow. While blood agar is not used to select for the growth of certain microbes, this type of agar is used to distinguish microbes that lyse red blood cells and degrade hemoglobin from those that don’t. How can you tell? By a color change in the agar.
Types of Hemolysis
When a bacterium produces and secretes toxins called hemolysins into the medium, they break down the red blood cells causing a color change visible on the agar plate. Bacteria aren’t the only microbes that produce hemolysins. Some fungi also do too.
Below are the different types of hemolysis, which produce distinct visible effects on blood agar.
Alpha Hemolysis
- Effect on blood agar: Green-grey or brown color around the bacterial colony caused by incomplete lysis of the blood.
- Mechanism: Hydrogen peroxide produced by bacteria turns hemoglobin to methemoglobin causing the green color of the medium. The cell’s membrane remains generally intact.
- Produced by: Staphylococcus aureus, Streptococcus pneumoniae
Beta Hemolysis
- Effect on blood agar: Clearing of the agar around the colony due to complete red blood cell lysis. Another type of hemolysis, alpha prime hemolysis (or wide-zone alpha hemolysis), could be confused for beta hemolysis. In alpha prime hemolysis, the agar around the bacterial colony has a small zone of unlysed red blood cells and complete lysis surrounding this.
- Mechanism: Beta hemolysis is caused by exotoxins called streptolysins which interact with cholesterol in cell membranes of the red blood cells to form pores.
- Produced by: Streptococcus pyogenes, Streptococcus agalactiae
Gamma Hemolysis
- Effect on blood agar: No effect.
- Mechanism: Gamma hemolysis is the absence of hemolysis activity.
- Produced by: Neisseria meningitidis, Enterococcus faecalis
Chocolate Agar: Chocolate… or Blood?
Aside from the “typical” blood agar plate described above, there’s another type of blood agar: chocolate agar. But unlike the name suggests, this type of agar is not made of chocolate. The agar takes on a chocolate color due to the addition of lysed blood into the medium. It’s used to grow organisms like Haemophilus influenzae, which require factor V (nicotinamide adenine dinucleotide) from the lysed red blood cells for growth. So sorry to those of you hoping for the tasty aroma of chocolate emanating from these plates!
Further Reading
Blood Agar Plates and Hemolysis Protocols. ASM. 2005.
Blood Agar – Composition, Principle, Preparation, Uses and Hemolysis. Microbe Notes. 2022.
Featured image: Pseudomonas aeruginosa growing on blood agar after 24 hours (left) and 48 hours (right) and breaking down blood cells. Source.
