Introduction
Blood Agar (BA) is an enriched and differential culture medium widely used in microbiology laboratories. It is prepared by adding 5–10% sterile defibrinated blood (commonly sheep, horse, or human blood) to nutrient agar base.BA supports the growth of fastidious organisms and allows detection of hemolysis patterns (alpha, beta, gamma) that are useful for bacterial identification.
Media Type
Category: Enriched & Differential media
Purpose:
Supports fastidious bacteria (Streptococcus, Neisseria, Haemophilus).
Differentiates bacteria based on hemolysis patterns.
Composition & pH of Blood Agar
Blood agar = Nutrient agar base + sterile blood (5–10%)
| Component | Function |
|---|---|
| Peptone (5 g) | Nitrogen, amino acids, growth factors |
| Beef extract (3 g) | Vitamins, nitrogen, salts |
| Sodium chloride (5 g) | Osmotic balance |
| Agar (15 g) | Solidifying agent |
| Distilled water | Solvent |
| Defibrinated blood (50–100 mL) | Enrichment, hemolysis detection |
Final pH: 7.2 – 7.4 (at 25°C)
Sterilization
Nutrient agar base: Autoclave at 121°C for 15 minutes.
Blood: Not autoclaved; sterilized by filtration or collected aseptically.
Blood is added aseptically to molten agar cooled at 45–50°C (to avoid hemolysis).
Preparation of Blood Agar
Prepare nutrient agar base and sterilize.
Cool molten agar to 45–50°C.
Add 5–10% sterile defibrinated blood aseptically.
Mix gently without frothing.
Pour into sterile Petri dishes (20–25 mL/plate).
Allow to solidify.
Store plates at 2–8°C.
Hemolysis Patterns on Blood Agar
This is differential due to visible hemolysis patterns:
Alpha hemolysis (α):
Partial hemolysis → greenish discoloration.
Example: Streptococcus pneumoniae, Viridans streptococci.
Beta hemolysis (β):
Complete hemolysis → clear zone around colonies.
Example: Streptococcus pyogenes, Staphylococcus aureus.
Gamma hemolysis (γ):
No hemolysis → unchanged medium.
Example: Enterococcus faecalis.
Uses
Growth of fastidious organisms (Streptococcus, Haemophilus, Neisseria).
Differentiation based on hemolysis.
Isolation of respiratory pathogens.
Detection of hemolytic toxins (e.g., Staphlococcus aureus beta hemolysin).
As a base medium for enriched selective media (e.g., Chocolate agar).
Limitations
Some organisms may require additional enrichment factors (X & V factors for Haemophilus).
Expensive compared to nutrient agar.
Blood must be collected and handled under sterile conditions
Conclusion
BA is one of the most important enriched and differential media in microbiology. It not only supports the growth of fastidious organisms but also helps differentiate bacteria based on hemolysis patterns, making it essential in diagnosis of streptococcal and staphylococcal infections.
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