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Methods
Bacterial cultures of S. aureus and S. epidermidis were incubated at 37°C for 16 hours in tryptic soy broth. One tenth milliliter of each culture was placed in a glass tube containing 0.4 ml of citrated rabbit plasma (Lampmire Biological Laboratories). Both tubes, one containing S. aureus and the other containing S. epidermidis, were incubated in a 37°C waterbath for 24 hours. The coagulase-positive organism, S. aureus, caused the plasma to form a clot in the tube (see top tube in image) whereas, the coagulase-negative organism, S. epidermidis, did not (see bottom tube in image). There are two types of coagulase produced by most strains of S. aureus, bound coagulase also called clumping factor and free coagulase. Bound coagulase is attached to the bacterial cell wall and can enzymatically convert fibrinogen in plasma to insoluble fibrin and cause the bacterial cells to clump. In this particular test clumping of the bacteria is not seen. This particular test is a better measure of the organism's ability to produce extracellular or free coagulase. Free coagulase reacts with a globulin plasma factor (coagulase-reacting factor) to form staphylothrombin. Staphylothrombin then catalyzes the breakdown of fibrinogen to insoluble fibrin. The insoluble fibrin then forms the clot that is seen in the top tube in the image. The tubes were held at an angle and the image was obtained with an Olympus C-3040 3.3-megapixel digital camera.

Discussion
S. aureus is a common cause of skin (impetigo, boils, carbuncles, and folliculitis), soft tissue (cellulitis), bone (osteomyelitis), and joint (septic arthritis) infections . This species of Staphylococcus can also cause a number of toxin-mediated human illnesses such as food poisoning, scalded skin syndrome, and toxic shock syndrome. There are several other staphylococcal species that colonize the human. These species of Staphylococcus are less commonly associated with human disease but are frequent contaminants in samples taken from skin and soft tissue lesions. The coagulase test is one way to differentiate the highly pathogenic S. aureus from the other less pathogenic staphylococcal species on the human body. S. aureus is a coagulase-positive organism whereas all the other staphylococcal species that colonize humans are coagulase negative.

Figure 1: Coagulase Test (Enlarged view)

Figure 2: Coagulase Test (Labeled view)