COAGULASE TEST
This test distinguishes Staphylococcus aureus, which generates the coagulase enzyme, from S.epidermidis and S.saprophyticus, which do not.
Principle
By converting fibrinogen to fibrin, coagulase causes plasma to clot. Most strains of S.aureus produce two types of coagulase: free coagulase and coagulase-reacting factor. Free coagulase converts fibrinogen to fibrin by activating a coagulase-reacting factor present in plasma. The presence of a fibrin clot in the tube test indicates the presence of free coagulase.Attached coagulase (clumping factor) converts fibrinogen to fibrin without the need for a coagulase-reacting factor. In the fast slide test, clumping of bacterial cells can be identified. On all negative slide tests, it is normally suggested that a tube test be done. If the slide test result is unclear, or if the slide test is negative and the Staphylococcus has been isolated from a serious infection, a tube test must be conducted.
Requirements
Human plasma (ideally pooled) or rabbit plasma, undiluted. Before using the plasma, it should be let to warm up to room temperature.
EDTA (ethylenediamine tetra acetic acid) plasma or citrate anticoagulated blood are commonly used.
Note:Citrate-using organisms like Klebsilla can occasionally cause citrated plasma in the tube test to clot. Heparin can be added to citrated plasma to prevent this. Human plasma may potentially contain inhibitory chemicals that prevent coagulase tests from working properly. For both slide and tube tests, adequate controls must be provided.
Methods
1. slide test
Method for slide test (to detect bound coagulase)
On each end of a slide, or on two separate slides, place a drop of physiological saline.
To make two thick suspensions, emulsify a colony of the test organism in each of the drops.
Note:Colonies grown on mannitol salt agar aren't good for coagulase testing. The organism must be grown on nutrition agar or blood agar first.
To one of the suspensions, add a drop of plasma and gently mix it in. Within 10 seconds, look for the creatures to gather together.
Controls
Staphylococcus aureus is a positive coagulase control. Escherichia coli or Staphylococcus epldermids were used as negative coagulase controls.
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