The coagulase test identifies whether an organism produces the exoenzyme coagulase, which causes the fibrin of blood plasma to clot. Organisms that produce coagulase can form protective barriers of fibrin around themselves, making themselves highly resistant to phagocytosis, other immune responses, & some other antimicrobial agents. Staphylococcus aureus is the only significant human pathogen that produces the enzyme coagulase. Thus, it is a confirmatory test for Staphylococcus aureus & also helps to differentiate from non pathogenic S. epidermidis . The coagulase enzymes exist in two forms- free & bound coagulase.
The extracellular free coagulase is a heat labile enzyme secreted free into culture medium. It requires the cooperation of plasma factor called coagulase reacting factor(CRF) for its clotting action. CRF is present in rabbit & human plasma. Free coagulase has thrombin like activity & converts fibrinogen to fibrin. This causes the cells to agglutinate or clump together which creates a lumpy look of positive coagulase test. The free coagulase is detected by appearance of fibrin clot in test tube.
Bound coagulase is heat stable constituent of cell wall which reacts directly ie independent of CRF, with fibrinogen & causes clotting. Hence, the cell culture of bacteria form an agar medium emulsified in a drop of saline on slide mixed with human or rabbit plasma gives rapid clumping of suspension in positive strains. The coagulase test can be done by test tube or slide test. Although both test tube and slide test can be performed for coagulase enzyme, tube test is more accurate. Thus tube test must be performed for all negative slide test before concluding as negative.
In the clinical laboratory, coagulase activity is used to distinguish between pathogenic (coagulase ) and nonpathogenic coagulase negative Staphylococci.
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Cited By Kamal Singh Khadka
Assistant Professor In PU, RE-COST, PNC, LA,NA