The coagulase test identifies whether an organism produces the exoenzyme coagulase, which causes the fibrin of blood plasma to clot. Organisms that produce catalase can form protective barriers of fibrin around themselves, making themselves highly resistant to phagocytosis, other immune responses, and some other antimicrobial agents.
The coagulase slide test is used to identify the presence of bound coagulase or clumping factor, which is attached to the cell walls of the bacteria. Bound coagulase reacts with the fibrinogen in plasma, causing the fibrinogen to precipitate. This causes the cells to agglutinate, or clump together, which creates the “lumpy” look of a positive coagulase slide test. You may need to place the slide over a light box to observe the clumping of cells in the plasma.
The coagulase tube test has been set up as a demo for you to observe in class. This version of the coagulase test is used to identify the presence of either bound coagulase or free coagulase, which is an extracellular enzyme. Free coagulase reacts with a component of plasma called coagulase-reacting factor. The result is to cause the plasma to coagulate. In the demo, the coagulase plasma has been inoculated with Staphylococcus aureus and Staphylococcus epidermidis and allowed to incubate at 37˚C for 24 hours. Staphylococcus aureus produces free coagulase; Staphylococcus epidermidis does not.
The coagulase test is useful for differentiating potentially pathogenic Staphylococci such as Staphylococcus aureus from other Gram positive, catalase-positive cocci.
The slide test should be read very quickly, as false positives can occur.
The slide test should not performed with organisms taken from high-salt media such as Mannitol Salt Agar, as the salt content can create false positives.
The tube test is more reliable than the slide test.
We generally don’t use the coagulase test when identifying unknowns.