Coagulase Test

Theoretical Background / Context

• 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.

Coagulase Test Principle of Work

• The coagulase test identifies whether an organism produces this exoenzyme as part of biochemical tests for bacteria.
• 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 through staphylococcus aureus coagulase test.