Medical Microbiology [Coagulase Test]

Coagulase test is used to differentiate between Staphylococcus aureus and other coagulase negative species. This test is based on the coagulative properties of S. aureus with plasma. This test is usually performed if the latex agglutination test for S. aureus is invalidated or equivocal.

S. aureus produces 2 forms of coagulase – free and bound. Different types of tests are used to observe the coagulation reactions, depending on the form of coagulase targeted.

Bound coagulase, also known as “clumping factor”, is detected using the slide test. This form of coagulase is bound to the bacterial cell wall, hence its name. It reacts with fibrinogen in plasma to form visible clumps. However this test is not performed in the lab anymore.

The tube test is done on free coagulase. Free coagulase released by the bacterial cell is often found in culture filtrates. It exists as a free form, unlike the bound coagulase attached to the cell. The free coagulase acts on prothrombin, producing a complex with similar properties to thrombin - staphylothrombin. This substance then acts on fibrinogen to form a fibrin clot in plasma.

1. 2-5 staphylococcal colonies are suspended in 0.5ml of rabbit plasma w/EDTA.
2. Incubate aerobically at 35^oC for 4 hours.
3. Check for clot formation.
4. If negative, continue incubation overnight and check again after the extended incubation.

Positive (+ve)	Visible clumps suspended in plasma
Negative	Absence of clumping

NOTE: Care should be taken when examining the clot formation. If too much time has passed before examining the clot formation, it could produce false negative results. This is due to the ability of certain S. aureus to produce staphylokinase, which breaks down any clot that has formed.

References:

"http://www.hardydiagnostics.com/catalog2/hugo/CoaguStaph.htm"

Also taken from http://en.wikipedia.org/wiki/Coagulase on 27 Sep 2009.

EDIT: By the way this entry is from MUNA.
teehee. XD