Methicillin Resistant Staphylococcus Aureus, The Coagulase Test

METHICILLIN RESISTANT STAPHYLOCOCCUS AUREUS, THE COAGULASE TEST
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Etiological agent: Methicillin-resistant Staphylococcus aureus
Microbial Classification
* Gram-positive bacteria with an approximate diameter of 0.5 – 1.0 µm.1,2
* Mostly grow in clumps, but can also be found growing in pairs and occasionally in short chains.
* Has resistance to methicillin, a beta-lactam antibiotic.
Virulence Factors
* Surface proteins known to colonize cells and tissues of the host organism.
* Inhibitory factors – these majorly affect the process of phagocytosis in the host organism, especially for Ig A (immunoglobulin protein A) and bacterial capsules.
* Toxins – these usually damage the host tissues and lead to the development of symptoms of disease. The toxins produced include α-toxin, β-toxin, δ-toxin, and the ϒ and leukocidin toxins.
Clinical Signs
* Acute skin and tissue infections – these include boils and abscesses; deep-seated tissue infections, including osteomyelitis; furunculosis; food poisoning; and toxic shock syndrome.1,3,4
The Coagulase Test
The test is mainly performed as a differentiating factor between S. aureus and other coagulase-negative staphylococci (CNS). The test appears relevant because Staphylococcus aureus produces coagulase, an extracellular protein, known to bind to host’s prothrombin. The binding of coagulase to prothrombin creates a complex, staphylothrombin, which in turn activates the protein-clipping ability of thrombin in the blood. Thrombin clips fibrinogen – a precursor protein – resulting to the formation of fibrin. This forms the basis of tube coagulase test, where blood plasma is incubated with S. aureus up to four hours in a test tube as follows:
Figure 1: Coagulase Tube Test photo courtesy of Hardy Diagnostics.
The specimens required in this test include: blood plasma obtained from a rabbit or horse and the bacterial culture to be tested. The culture should specifically be grown in an agar broth. In the tube test, positive confirmation of S. aureus is noted due to the formation of a clot. The absence of the clot signifies absence of S. aureus in the samples collected.5,6 The Coagulase test is important in initial screening and confirmatory tests regarding identification of Staphylococcus aureus.
A second coagulase test, the coagulase slide test, is used in screening for S. aureus isolates in collected samples. It specifically measures bound coagulase, unlike the tube test which measures free coagulase present. One spo...