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Koch’s Postulates:
A Logical Mechanism for Determining the Cause of
Diseases with Near Certainty
Alisha John
6/11/2009
Finding a cure for any disease – cancer, AIDS, diabetes, HIV, heart disease, etc. – starts
with the same process, determining the causative agent. This is often easier said than done, but
if the pathogen (disease causing agent) is identified, it makes the ailment much easier to
research and eventually cure. Koch’s postulates form a logical mechanism that provides
researchers a means to identify the microorganism responsible for a specific disease.
Koch’s postulates were developed by Robert Koch in the late nineteenth century and
have been the golden standard in identifying pathogens ever since. Koch used these postulates
to prove that Mycobacterium tuberculosis caused tuberculosis, which was responsible for
approximately one-seventh of all reported human deaths at that time. Overall, Koch’s
postulates state:
1) The suspected pathogenic organism should be present in all cases of the disease. In
addition, the organism should be absent from all healthy animals.
2) The suspected organism should be grown in pure culture, a laboratory grown culture
containing only one type of microorganism.
3) If a healthy animal is inoculated with cells from the pure culture, that is the cells are
introduced to the animal’s body, the animal should become diseased.
4) The pathogenic organism should be isolated from the newly infected animal, grown in
laboratory culture and be shown to be identical to the original organism.
The first step in proving a certain microorganism causes a disease is to observe infected
samples (blood, tissue, etc.) to determine a suspected pathogen. This is done by observing the
samples under a microscope; different staining techniques may be used to allow better
differentiation between cells. Healthy samples should also be examined since they will not
contain the microorganism; thus, if the suspected microorganism is present in the healthy
animal it can be concluded that it is not the pathogen for the target disease.
Once a suspected organism is detected, it needs to be isolated in a laboratory culture. This
involves growing a pure culture on a media. A culture media is a solution of various nutrients,
such as carbon, nitrogen, vitamins, sugars and amino acids, suitable for the growth of
microorganisms. There are many different types of media, including solid, semisolid and liquid.
Each of these types can also be defined or complex. Solid, semi-solid and liquid are terms used
to describe the consistency of the media; both solid and semi-solid media contain a type of
solidifying agent such as agar or gelatin. The difference between defined and complex media is
more important. Defined is used to describe a media in which the exact composition is know.
On the contrary, complex is used to describe media in which the exact composition is not know;
these usually contain plant or animal products (blood, phytone, peptone, etc.) of which the
concentrations of substances is not known precisely. Choosing the proper media is essential
because different microorganisms will grow in certain conditions, but not others.
Once the organism has been successfully grown in the laboratory, it must be inoculated or
introduced to a healthy test animal. If the animal develops the disease, the microorganism may
still be the causative agent, if it does not cause the disease, it is not.
If the suspected pathogen caused disease in the test animal, it must then be reisolated from
the animal, grown in culture and examined. If the microorganism is determined to be the same
as the original organism used to inoculate the healthy animal then it can be concluded that it is
the causative agent of the disease in question.
An overview of Koch’s postulates is provided in Figure 1 below.
Figure 1: Koch’s Postulates for proving that a specific microorganism causes a specific disease. Source: Madigan, Martinko,
Dunlap and Clark, p. 15 (2009).
Overall, Koch’s Postulates provide a logical mechanism that allows researchers to
determine, with an unparalleled amount of certainty, that a certain microorganism causes a
specific disease. Ultimately, this helps researchers understand the behavior of diseases and aids
in the discovery of cures and vaccines. However, it must be stated that it is sometimes
impossible to satisfy all of Koch’s postulates. For example, many human pathogens do not
affect other animals the way they do humans, thus, it cannot be shown that the microorganism
causes disease in a healthy individual since it is unethical to subject humans to the necessary
testing. For such cases, clinical research and disease tracking are used to provide near definite
proof of the pathogen-disease partnership. In addition, not all diseases are caused by
microorganisms, the vast majority are, but Koch’s postulates cannot be used to determine the
causative agents of those diseases. Finally, the logical mechanism underlying Koch’s postulates
can be used in other applications; anytime a problem needs to be attributed to a certain
characteristic, the fundamentals of Koch’s postulates can be used. As such, Koch’s postulates
provide a logical mechanism not only for biological researchers, but also for anyone who
desires to determine the causative agent of a given problem.
Bibliography
Leboffe, M. J., & Pierce, B. E. (2006). Microbiology Laboratory Theory and Application. Morton Publishing
Company.
Madigan, M. T., Martinko, J. M., Dunlap, P. V., & Clark, D. P. (2009). Brock: Biology of Microorgansism.
San Francisco: Pearson Education, Inc.