their clinical pathology manual, Clinical Diagnosis by Laboratory Methods,
Todd, Sanford and Wells (24) noted that both coagulase tests are reliable
methods of distinguishing between pathogenic strains and nonpathogenic
strains, but the ASM Manual of Microbiological Methods was published in
1957 (1) without mention of coagulase. In the first edition of Bailey and
Scott in 1962 (2), the tube coagulase test was the accepted method to
identify a pathogenic Staphylococcus. Today, even though a variety of
diagnostic tests are available to identify S. aureus in the clinical
microbiology laboratory, the coagulase tests remain simple, reliable
methods of identification. It is such a standard test that newly developed
identification methods are evaluated by comparison with the tube
coagulase test.
As it became more common to rely on coagulase as a test to identify S.
aureus, researchers studied the characteristics of the coagulase
reaction. Chapman, Berens and Stiles (9) and Turner and Schwartz (25)
both examined the effect on coagulase of factors such as the species of
organism donating plasma, the age of the inocula, the presence of
anticoagulants, and the amount of salt. Researchers utilized a wide
variety of plasmas to observe the activity of coagulase. For example,
Chapman (10) felt that whole human blood worked best, while Orth,
Chugg and Anderson (20) determined that human plasma was the best
sera, while bovine, chicken, and pig were the worst. Yrios (27) found
that heparinized pig plasma had results similar to commercial rabbit
plasma. Ethylenediamine tetra-acetate (EDTA) was found to give
superior results to citrated plasma (27). Bailey and Scott noted that,
while various researchers were using plasma from a variety of species
with differing results, consistent results were obtainable by using
commercially available dehydrated rabbit plasma (2). Several chemicals
are commonly used to prevent activation of the clotting cascade when
blood is drawn. These include heparin, citrate, and ethylenediamine
tetra-acetate. Heparin treated blood is no longer used for the coagulase
test, and while citrate-treated blood can be used, it is possible that the
organism being tested might utilize citrate metabolically, removing it
from the solution and initiating the clotting cascade (12). For this
reason, most plasma used for the coagulase test is treated with EDTA,
which functions by chelating divalent cations such as Ca
2+
which are
required for enzymes to function.
While Staphylococcus species give similar results in the two coagulase
tests, tube and slide, evidence began to accumulate that there were two
different mechanisms of action. Duthie (11) found that heated and
cooled plasma could not be used to form a clot in the tube test, although
it retained activity in the slide clumping test. Duthie partially purified the
clumping factor, the cell-bound protein which is now referred to as bound
coagulase. This protein acts on fibrinogen to cause clumping of the
bacterial cells. O’Connell et al. (19) identified bound coagulase as an
adhesin of the microbial surface components recognizing adhesive matrix
molecules (MSCRAMM) family. The secreted protein, staphylocoagulase,
binds to prothrombin. Hemker, Bas and Muller (15) determined that,
although this enzyme acts like thrombin, it is not. It has a different
amino acid composition, it chromatographs differently, and it is not
inhibited by heparin. Panizzi et al. (21) indicate that fibrinogen is