The Coagulase Test
By: Jenni Asterino
Introduction
In the medical world, one of the most important parts of diagnosing a patient is the work done in a lab
on an individual’s samples. The hospital lab consists of many departments which test everything from
hematology to chemistry. However, one of the most beneficial departments, when it comes to a
person’s bodily fluids, is the Microbiology department. This is where tests which are specifically used to
identify the pathogenic bacteria that are making people sick are performed. These tests help doctors
pinpoint their diagnosis, and are indicators as to which antibiotics will be effective, if any at all. One
such test is to detect the presence of coagulase. The coagulase test is a method of identifying and
differentiating bacteria, specifically Staphylococcus, through detection of the enzyme coagulase.
What is coagulase?
In order to understand how the coagulase test works, you first have to understand what coagulase is.
Coagulase is an enzyme produced by certain bacteria which causes plasma in blood to coagulate or
curdle. The normal mechanism for the coagulation of plasma consists of the protein Prothrombin being
broken down into Thrombin, an enzyme which degrades proteins1. Thrombin then catalyzes (speeds up)
the conversion of the protein fibrinogen into fibrin (figure 1). Fibrin is a sticky protein that groups
together to form blood clots. This mechanism is what prevents people and animals from bleeding out
after an injury.
PROTHROMBIN
↓
THROMBIN
↓
FIBRINOGEN → FIBRIN
Figure 1: the mechanism for plasma coagulation
Bacteria use coagulase to take advantage of this mechanism and thereby increase their chances of
survival in the human body in two different ways:
1. Coagulase can be excreted by bacteria straight into the bloodstream. This type of coagulase,
called free coagulase, complexes with Prothrombin in the blood. It forms a protein degrading
enzyme, the name varies based off the bacterial species, which takes the place of Thrombin and
converts Fibrinogen into Fibrin2. The bacteria can then hide themselves in the clump of fibrin
and move undetected through the body.
2. Coagulase can also be bound to the surface of the bacterial cell wall. This is called bound
coagulase or clumping factor. This clumping factor can directly convert Fibrinogen into Fibrin.
Certain bacteria also are covered in Protein A which works in conjunction with the clumping
factor. Protein A attaches to the Fc (unchanging) region of antibodies, forcing them to point in
the wrong direction. The human body then recognizes the antibodies as not being foreign, and
doesn’t attack the bacteria- thinking it is one of our cells2. This, along with the fibrin meshwork
produced by clumping factor, allows the bacteria to evade cell death and stick themselves to
traumatized tissue and blood clots.
How is coagulase used to identify and differentiate bacteria?
Bacteria can not all produce the enzyme coagulase. In fact, different bacteria within the same genus can
possess varying abilities to produce coagulase, and sometimes, it can even come down to variations
among different strains of the same species. This selectiveness in coagulase production, allows us to
differentiate and identify bacteria in hospital and laboratory settings through the coagulase test. There
are two different types of coagulase tests based on the two different types of coagulase produced by
bacteria:
1. The coagulase tube test is mainly used to identify the presence of free coagulase. In this test,
colonies of the bacteria are mixed with modified rabbit serum (plasma). If the bacteria produce
coagulase, it will break the fibrinogen down into sticky fibrin, and an observable clump will
precipitate in the tube2 (figure 2). The tube test can also be used on bacteria suspected of
having bound coagulase, as further explained below.
2. The coagulase slide test is used to identify the presence of bound coagulase or clumping factor,
and protein A if it is present. In this test, bacterial colonies are mixed with a reagent which
contains latex beads covered in fibrinogen and antibodies, specifically IgG. If bound coagulase is
present, it will convert fibrinogen to fibrin and cause the latex particles to clump. If protein A is
present it will bind the Fc region of IgG and clump the bacterial cells to the beads. The clumping
will be seen as a lumpy, granulated appearance on a slide2 (figure 3). Although the slide test is
quicker and cheaper, some bacteria will display false negatives on the slide which the tube test
can positively identify.
Figure 2: Coagulase Tube Test: Notice the solid precipitate
In the top tube which indicates the presence of coagulase.
The bottom tube is negative and the serum stayed liquid3
Figure 3: Notice the granulated, lumpy appearance of the
clumped beads in block one indicating coagulase. Block two
shows a negative coagulase reaction4
Which bacteria are identified and differentiated through this test and why are
they relevant?
There are several microorganisms that are able to produce coagulase, but most of these are relatively
harmless. The main pathogenic microorganism that produces coagulase, and the main reason for using
the coagulase test in medical settings, comes from the genus Staphylococcus.
Staphylococci overall are harmless. They are part of the body’s microbiome (normal flora), and are
commonly found on skin and mucous membranes. However, there is one species of Staphylococcus
which is a primary human pathogen; Staphylococcus aureas. This bacterium causes life-threatening
diseases such as pneumonia, septicemia (blood infections), endocarditis and abscesses2. The pathogenic
nature of S. aureas makes its identification and differentiation from other Staphylococcus species all the
more important.
One important characteristic of S. aureas is that it’s the only human pathogen from the genus that is
coagulase positive. There are only five other Staph species that produce coagulase; however, they are
all non-pathogenic to humans2. This allows S. aureas to easily be differentiated from other pathogenic
Staphylococci through the use of the coagulase tests described above.
Conclusion
The world is full of bacteria and while most are just a natural part of our lives (and bodies), there are
some that can seriously cause us harm and even kill us. Luckily, we now have the technology to identify
these bacteria through their biochemical composition in the form of enzymes. This allows us to
differentiate between species and find the exact pathogenic bacterium. The enzyme coagulase, in
particular, helps us to identify and differentiate Staphylococcus aureas from other Staphs through the
coagulase tests. These tests, in the form of a tube or a slide, ensure people get prescribed the correct
antibiotics and help us avoid the life-threatening diseases that Staphylococcus aureas can cause. As
technology progresses, these tests will become more accurate; allowing bacteria to be differentiated
down to a strain level and save more lives.
References
1: http://www.nlm.nih.gov/cgi/mesh/2011/MB_cgi?mode=&term=Coagulase
2: Microbiology 422 Lab Manual. Dr. Sillman. Spring 2014. Penn State Multimedia and Print Center.
2014.
3: http://pathinformatics.com/microbiology/saeI/Ia.htm
4: https://catalog.hardydiagnostics.com/cp_prod/content/hugo/StaphTexBlueKit.htm