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Blood agar haemolysis is used to determine the haemolytic ability of some pathogenic microorganisms including Streptococcus pneumoniae, Staphylococcus aureus and Micrococcus species. Some pathogenic bacteria are capable of expressing some specific exotoxins known as haemolysins – which react as antibodies homologous to the surface antigens of red blood cells (erythrocytes). This phenomenon allows these organisms the exceptional ability to lyse erythrocytes in blood culture media, and this can serve as basis for their identification in the microbiology laboratory. Pathogenic microorganisms produce three (3) different types of haemolysis on blood agar culture media; and they are: 1. alpha (α) haemolysis, 2. beta (β) haemolysis and 3. gamma (γ) haemolysis (Figure 1). Alpha haemolysis is a partial lysis of red blood cells (RBCs), and this is seen in the cultures of Streptococcus mitis and S. pneumoniae which both show a green-opaque colony on blood agar plates. Beta haemolysis is a complete lysis of RBCs, and it is mostly seen with cultures of Streptococcus pyogenes, Staphylococcus aureus and Streptococcus agalactiae.In β-haemolysis, a clear zone normally surrounds the bacterial colony. Gamma haemolysis produces no clear-cut lysis of RBCs, and the bacterial colonies are normal-looking colonies because no haemolysis occurred. Micrococcus species and Staphylococcus epidermidis are typical examples of γ-haemolytic bacteria. 


  1. Blood agar haemolysis test is carried out on nutrient agar modified by the incorporation of 5 % oxalated or defribinated blood (from horse, sheep, and rabbit or ox blood) and 1 % sodium chloride.
  2. Prepare and sterilize nutrient agar medium according to the manufacturer’s instructions.
  3. After sterilization, liquefy agar medium by warming in a water bath at 50oC.
  4. Add 0.5-10 ml of defribinated blood to the nutrient agar media and mix thoroughly by rotation or inversion of the flask containing the prepared medium.
  5. Aseptically pour sterile blood agar medium into Petri dish(s), and allow on the workbench to solidify.
  6. Aseptically inoculate the test bacteria on the prepared blood agar plate and incubate plate(s) at 37oC for 18-24 hrs.
  7. Observe plate(s) for the presence of haemolysis (α, β, or γ) after incubation.
Figure 1. Haemolysis on blood agar culture plates. Photo courtesy:

Further reading

Brooks G.F., Butel J.S and Morse S.A (2004). Medical Microbiology, 23rd edition. McGraw Hill Publishers. USA.

Goldman E and Green L.H (2008). Practical Handbook of Microbiology, Second Edition. CRC Press, Taylor and Francis Group, USA.

Madigan M.T., Martinko J.M., Dunlap P.V and Clark D.P (2009). Brock Biology of Microorganisms, 12th edition. Pearson Benjamin Cummings Inc, USA.

Mahon C. R, Lehman D.C and Manuselis G (2011). Textbook of Diagnostic Microbiology. Fourth edition. Saunders Publishers, USA.

Patrick R. Murray, Ellen Jo Baron, James H. Jorgensen, Marie Louise Landry, Michael A. Pfaller (2007). Manual of Clinical Microbiology, 9th ed.: American Society for Microbiology.

Wilson B. A, Salyers A.A, Whitt D.D and Winkler M.E (2011). Bacterial Pathogenesis: A molecular Approach. Third edition. American Society of Microbiology Press, USA. Woods GL and Washington JA (1995). The Clinician and the Microbiology Laboratory. Mandell GL, Bennett JE, Dolin R (eds): Principles and Practice of Infectious Diseases. 4th ed. Churchill Livingstone, New York.

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