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Coagulase is an enzyme produced by Staphylococcus aureus that converts fibrinogen to fibrin. In the laboratory, it is used to distinguish between different types of Staphylococcus isolates. Coagulase negativity excludes S. aureus. (That is to say, it is coagulase-positive.)

Coagulase reacts with prothrombin in the blood. The resulting complex is called staphylothrombin, which causes blood to clot by converting fibrinogen to fibrin. Coagulase is tightly bound to the surface of the bacteria S. aureus and can coat its surface with fibrin upon contact with blood. It has been proposed that fibrin-coated staphylococci resist phagocytosis making the bacteria more virulent. Bound coagulase is part of the larger family of MSCRAMMS

The coagulase test is used to differentiate Staphylococcus aureus from the other species of Staphylococcus. The test uses rabbit plasma that has been inoculated with a staphylococcal colony. The tube is then incubated at 37 degrees Celsius for 1-1/2 hours. If positive (i.e., the suspect colony is S. aureus), the serum will coagulate, resulting in a clot (sometimes the clot is so pronounced that the liquid will completely solidify). If negative the plasma remains liquid. The negative result may be S. epidermidis but only a more detailed identifcation test can confirm this. Using biochemical tests as like in API™ tests and BBL CRYSTAL™ methods.

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