Bacterial Coagulase Test: Introduction, Principle, Procedure, Result Interpretation, Limitations and Keynotes
Introduction of Bacterial Coagulase Test
The bacterial coagulase test is an enzymatic test that is used to differentiate Staphylococcus aureus (positive) from coagulase-negative staphylococci (CoNS). Coagulase is an enzyme produced by S. aureus that converts soluble fibrinogen of plasma to insoluble fibrin. S. aureus produces two forms of coagulase i.e. bound and free coagulase.
Slide coagulase test is proceeded to detect bound coagulase or clumping factor.
A tube coagulase test is performed to detect free coagulase.
Principle of Bacterial Coagulase Test
Staphylococcus aureus is the most pathogenic species among staphylococci. It is different from other species by the presence of coagulase. This enzyme is a thermostable thrombin-like substance that activates ﬁbrinogen to form ﬁbrin, resulting in a ﬁbrin clot. This is demonstrated in the test tube by the formation of a clot when plasma is inoculated with Staphylococcus. The substance is known as free coagulase ( detectable in tube coagulase test) since it is liberated by the cell. In most, but not all, S. aureus organisms, ﬁbrinogen binding cell surface receptor is also present in the cell wall, called bound coagulase’ or clumping factor ( detectable on slide coagulase test). The clumping factor is demonstrated by the ability of the organism to act directly on the ﬁbrinogen in the plasma to clump it in a slide assay. The test for clumping factor is rapid but requires several colonies and this factor is not present in all S. aureus organisms. In addition, the clumping factors can be masked by cell surface capsular polysaccharides and can be present in other species, namely, Staphylococcus lugdunensis and Staphylococcus schleiferi and the reason, negative slide tests must be followed with a conﬁrmatory tube test.
Note: Rabbit plasma is preferred rather than human plasma due to the following reasons:
Human plasma for the test is less sensitive.
It is potentially infectious with human pathogenic viruses.
Loops or sterile sticks
Glass or plastic tubes
clean and grease-free glass slides or black coated cards
Quality control strains
Positive Control (PC): Staphylococcus aureus ATCC 25923
Negative Control (NC): Staphylococcus epidermidis ATCC 12228
Procedure for slide Coagulase Test Method
It is for the detection of bound coagulase.
Put a drop of distilled water on each end of a slide or you can also put it on two separate slides.
Emulsify a colony of the test organism (To avoid misidentiﬁcations, only perform this test on classic-looking white to yellow, creamy, opaque, hemolytic colonies of gram-positive cocci in clusters that are catalase-positive. Hemolysis should only be observed on a fresh blood agar plate at equal and less than 18 hours) in each of the drops to make two thick suspensions. Note: Avoid taking colony from mannitol salt agar culture because of not suitable for coagulase testing. The organism must ﬁrst be cultured on nutrient agar or blood agar.
Add just a loopful of plasma to one of the suspensions, and mix gently.
Look for clumping of the organisms within 10 seconds, No plasma is added to the second suspension.
This is used to differentiate any granular appearance of the organism from true coagulase clumping or agglutination.
Procedure for Tube Coagulase Test Method
Take 0.5 ml of plasma which is diluted 1 in 5 with saline. If there is stored plasma, bring it to room temperature (25°C).
Inoculate with one colony of staphylococcus growing on a non-inhibitory medium.
Incubate the tube at 37°C without CO2 for up to 4 hours and observe for clot formation hourly. Do not agitate the tube during observations; rather, gently tilt to observe the clot.
If the clot is not observed at that time, reincubate the tube at room temperature and read after 20 hours.
Observations of Bacterial Coagulase Test
In a positive slide test, evoke clumping of the organism shows the presence of the bound coagulase whereas in a positive tube test, the plasma in the tube clot and does not flow when the tube is inverted. One thing we should know; on continued incubation, the clot may be lysed by fibrinogen secreted by some strains.
Result and Interpretation of Bacterial Coagulase Test
For slide coagulase test
Positive: Clumping or agglutination within 10 secs
Negative: No clumping within 10 secs
For tube coagulase test
Positive: Clot formation
Negative: No clot formation
All strains producing negative slide tests must be tested with the tube coagulase test. If there is any degree of the clot, the test should consider positive.
List of coagulase-positive staphylococci
Note: among them only slide positive organisms-
whereas only tube coagulase test positive-
Limitations of Coagulase Test
Coagulase testing cannot be performed from growth on mannitol salt agar.
Methicillin-resistant S. aureus (MRSA) can be deﬁcient in bound coagulase, which results in a negative slide test.
S. intermedius and S. hyicus may be positive in the tube test.
Do not use citrated blood, as false-positive results can occur.
Keynotes on Bacterial Coagulase Test
Do not practice leaving the test at 37°C for more than 4 hours, since S. aureus ﬁbrinolysin can lyse the clot.
If 4 hours of incubation at 37°C is inconvenient for stafﬁng, the test is most sensitive when incubated at room temperature(25°C) for the entire time, but the clot may take longer to form.
Alternatively, if the tube is left at 37ºC, add a drop or two of 5% CaCl2 at 24 h. If a clot forms, the isolate is coagulase-negative; if a clot does not form, ﬁbrinolysin has lysed a previously formed clot and this result conﬁrms the isolate as coagulase positive but the reference for this article is unpublished data of H. D. Isenberg, editor in chief Clinical Microbiology Procedures Handbook, original and second editions.
The coagulase clot can be destroyed by S. aureus ﬁbrinolysin or staphylokinase, a plasmid-carried enzyme which is more active at 35°C than at 25°C.
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