- Coagulases are enzymes that clot blood plasma by a mechanism that is similar to normal clotting.
- The coagulase test microbiology 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.
