Staphylococcus aureus possesses proteins ("coagulase") which can convert fibrinogen to fibrin and cause plasma to clot. These are absent in other species of Staphylococci.
Types of coagulase:
(1) bound to cell wall: detected with a slide test
(2) free: detected with a tube test
Reagent: rabbit plasma anticoagulated with EDTA
Test
Medium
Testing
Positive if
slide
plasma mixed with a suspension of bacteria in saline
read within 2 minutes
agglutination of suspension (often within 20 seconds)
tube
colony sample mixed with rabbit plasma in a sterile tube
incubate at 35°C for 4 hours, then overnight at 35°C if negative
clot formation
Both tests use positive and negative controls which should give expected results for a valid test.
If the slide test is positive, then the isolate is designated coagulase-positive Staphylococcus aureus.
If the slide test is negative, then the tube test is performed. If the tube test shows agglutination after a 4-hour incubation then the isolate is designated coagulase-positive Staphylococcus aureus.
If the 4-hour tube test is negative, then it is read again after overnight incubation. Fibrinolysins produced by Staphylococcus aureus may interfere with the early clot formation.
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