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Coagulase Test

Staphylococcus aureus, the most pathogenic of the staphylococci, is separated from other species by the presence of coagulase. Coagulase is a thermostable thrombin-like substance that activates fibrinogen to form fibrin, resulting in a fibrin clot. This is demonstrated in the test tube by the formation of a clot when plasma is inoculated with Staphylococcus. The substance is known as free coagulase since it is liberated by the cell. In most, but not all, S. aureus organisms, fibrinogen binding cell surface receptor is also present in the cell wall, called ” bound coagulase” or “clumping factor”. The clumping factor is demonstrated by the ability of the organism to act directly on the fibrinogen in the plasma to clump it in a slide assay. This test is used specifically to differentiate species within the genus. S. aureus (Usually Positive) from S. saprophyticus, S. epidermidis (negative). A positive coagulase test is usually the final diagnostic criterion for the identification of S. aureus.

Slide Coagulase Test

For the slide coagulase test, a drop of physiological saline was placed on three places of a slide, and then a colony of the test organism was emulsified in two of the drops to make thick suspensions. Later a drop of plasma was added to one of the drops to make thick suspensions. Then a clumping was observed within 10 seconds for the positive coagulase test. No plasma was added in the second suspension. This was used for the differentiation of any granular appearance of the organism from true coagulase clumping. The third drop of saline was used for a known strain of coagulase-positive staphylococci

Tube Coagulase Test

This test is carried out to detect the production of free coagulase. Plasma contains coagulase reacting factor (CRF) which activates free coagulase. The activated coagulase acts upon prothrombin thus converting it to thrombin. Thrombin converts fibrinogen into fibrin which is detected as a firm gel (clot) in the tube test. A tube test is performed when negative or doubtful results are obtained in the slide coagulase test.

Tube coagulase Test

Tube coagulase tests of bacteria showing negative and positive tests as shown above image.

Further Reading

1. Cowan & Steel’s Manual for identification of Medical Bacteria. Editors: G.I. Barron & R.K. Felthani, 3rd ed 1993, Publisher Cambridge University Press.
2. Bailey & Scott’s Diagnostic Microbiology. Editors: Bettey A. Forbes, Daniel F. Sahm & Alice S. Weissfeld, 12th ed 2007, Publisher Elsevier.
3. Clinical Microbiology Procedure Handbook, Chief in editor H.D. Isenberg, Albert Einstein College of Medicine, New York, Publisher ASM (American Society for Microbiology), Washington DC.
4. Colour Atlas and Textbook of Diagnostic Microbiology. Editors: Koneman E.W., Allen D.D., Dowell V.R. Jr, and Sommers H.M.
5. Jawetz, Melnick and Adelberg’s Medical Microbiology. Editors: Geo. F. Brook, Janet S. Butel & Stephen A. Morse, 21st ed 1998, Publisher Appleton & Lance, Co Stamford Connecticut.
6. Mackie and Mc Cartney Practical Medical Microbiology. Editors: J.G. Colle, A.G. Fraser, B.P. Marmion, A. Simmous, 4th ed, Publisher Churchill Living Stone, New York, Melborne, Sans Franscisco 1996.
7.  Textbook of Diagnostic Microbiology. Editors: Connie R. Mahon, Donald G. Lehman & George Manuselis, 3rd edition2007, Publisher Elsevier.