Diseases such as Lyme disease, poliomyelitis, and tuberculosis have well known etiology, “the set of factors that contributes to the occurrence of the diseases (Microsoft Encarta 2008).” Other diseases have an etiology that is not clearly understood – for example the association between the bacterium Helicobacter pylori and ulcer. For still others – like Alzheimers disease, the etiology is not yet known. This disease (Alzheimers) and other diseases like hemophilia, osteoarthritis, and cirrhosis are not caused by microorganisms but considered to be genetic and degenerative diseases. The diseases mentioned in the first sentence are of course caused by microorganisms - Borrelia burgdorferi for Lyme disease, poliovirus for poliomyelitis, and Mycobacterium tuberculosis for tuberculosis (Matthews 1998). How did scientists determine that the microorganisms are the causative agents for the infectious diseases? To answer this question, let us discuss Robert Koch’s infamous postulates known as Koch’s Postulates.
Understanding Koch’s Postulates
German physician Robert Koch played a major role in establishing the concept that microbes cause specific diseases. He published journal articles (Ingraham 2000) in 1877 presenting his experimental findings on the relationship of microorganism and the anthrax disease that kills sheep and cattle. He demonstrated that the anthrax bacterium (presently known as Bacillus anthracis) were always present in the blood of infected animals and absent in healthy animals. He claimed that the mere occurrence of bacteria in the animal blood did not prove that they had caused the illness; the bacteria could have been there as a consequence of the illness (Nester 2000). In order to back up this statement, he experimented more.
He drew blood sample from a diseased animal (anthrax disease) and injected it into a healthy one. The blood injected animal developed the same disease symptoms as the diseased animals and died after few days. He repeated this experiment many times and came up with the same results. (Note that the repeatability of experimental results is a key criterion in the validity of any scientific proof or evidence.) He further strengthened his findings on the etiology of anthrax by culturing the bacteria in solutions outside the animal’s body, and he showed that the bacterium would cause anthrax disease even after many culture transfers (Nester 2000; Ingraham 2000).
Using the experimental procedures he used in his anthrax experiment, he was able to demonstrate that the Mycobacterium tuberculosis is the causative agent of the disease, tuberculosis.
Koch’s research on anthrax and tuberculosis has provided the framework for the study of the etiology of transmittable diseases. At present, we call Koch’s experimental requirements as Koch’s Postulates which are summarized as follows (Ingraham 2002):
1. Similar pathogen must be present in every case of the disease.
2. The pathogen should be isolated from the diseased animal host and grown in pure culture.
3. A healthy laboratory animal inoculated with the pathogen taken from the pure culture must acquire the disease.
4. The pathogen must be isolated from the inoculated animal and should be the same from the microbe in the original animal.
Ingraham, John. 2000. Introduction to Microbiology. Pacific Grove, Calif.: Brooks/Cole Pub.
Matthews, Bernard. 1998. An Introduction to Microbiology. Cambridge University Press.
Microsoft Encarta. 2008. Dictionary. Microsoft Corporation.