Coagulases are enzymes that clot blood plasma by a mechanism that is similar to normal clotting. The coagulase test identifies whether an organism produces this exoenzyme. This enzyme clots the plasma component of blood. The only significant disease-causing bacteria of humans that produce coagulase enzyme are Staphylococcus aureus. Thus this enzyme is a good indicator of the pathogenic potential of S. aureus.
In a human host, the action of the coagulase enzyme produces clotting of the plasma by converting fibrinogen to fibrin in the immediate vicinity of the bacterium as a means of protection by itself. The fibrin meshwork that is formed by this conversion surrounds the bacterial cells or infected tissues, protecting the organism from non-specific host resistance mechanisms such as phagocytosis and the antistaphylococcal activity of normal serum. This enables the bacterium to persist in the presence of a host immune response, which can lead to the establishment of infection. Thus, coagulase is described as a virulence factor ( disease-causing factor) of Staphylococcus aureus. Citrate and EDTA are usually added to act as anticoagulants and prevent false-positive results.
Most strains of S.aureus produce one or two types of coagulase; free coagulase and bound coagulase. Free coagulase is an extracellular enzyme that reacts with prothrombin and its derivatives. Bound coagulase is localized on the surface of the cell wall and reacts with α- and β-chains of the plasma fibrinogens to form a coagulate. Free coagulase is an enzyme that is secreted extracellularly and bound coagulase is a cell wall-associated protein. Free coagulase can be detected in tube coagulase test and bound coagulase can be detected in slide coagulase test.
Slide coagulase test may be used to screen isolates of S.aureus and tube coagulase may be used for further confirmation. There are seven antigenic types of free coagulase, but only one antigenic type of bound coagulase exists. Free coagulase is always heated labile while bound coagulase is heat stable.
In the test, the sample is added to rabbit plasma and held at 37° C for a specified time. Clot formation -occurs within 4 hours is interpreted as a positive result and indicative of a virulent Staphylococcus aureus strain. The absence of coagulation after 24 hours of incubation is a negative result, indicative of an avirulent strain.
