The fibrinolysin test is used to determine the presence of a fibrinolytic enzyme which can dissolve fibrin clots. The fibrinolysin (a.k.a., staphylokinase) produced by most strains of Staphylococcus aureus, as well as, the streptokinases produced by virulent group A b-hemolytic Streptococcus (Streptococcus pyogenes) are examples of fibrinolytic enzymes, but are antigenically and enzymatically distinct from each other. The group C streptococci also produce an antigenically distinct fibrinolytic streptokinase and it is this particular enzyme that has been exploited commercially as the source of a thrombolytic (clot-busting) enzyme for clinical use in humans.
Procedure: Two tubes will be prepared, one without any bacterial inoculum and the other with S. epidermidis to demonstrate the effects of a non-fibrinolysin producer. These tubes will be compared to the results obtained with the S. aureus strain, following prolonged incubation of the coagulase test, which will serve as an example of a positive fibrinolysin producer.
In the first tube, CaCl2 (40 mmol/cc plasma) is added to ~0.5cc of platelet-rich plasma (~ >150 x 106 platelets/cc plasma) to produce a fibrin clot. The second tube is prepared identically, except that a generous loopful of the S. epidermidis strain is resuspended in the CaCl2-treated plasma. Thoroughly homogenize the inoculum with the loop and incubate the tube at 37o C for one to four hours. Examine the tube at 30 minute to hourly intervals for the first couple of hours for the presence of a clot by tipping the tube gently on its side.
Reincubate the tube overnight to see if the clot subsequently lyses. In strains that produce fibrinolysin, the clot will be slowly digested.
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