November 21, 2010

Koch's Postulates

In my last post I was describing how scientists use the scientific method to keep their research objective. One of the best examples of applying the scientific method are Koch's Postulates. Robert Koch (1843-1910, left) was a german physician who was interested in how diseases are spread. He is considered the founder of modern microbiology and bacteriology together with his contemporary, Louis Pasteur (1822-1895, right).

In the course of his career, he developed countless microbial techniques that are still in use today. The Petri dish is named after his assistant. He identified the causative agents of Tuberculosis (Mycobacterium tuberculosis), Anthrax (Bacillus anthracis) and Cholera (Vibrio cholera). This earned him the Nobel prize for medicine in 1905.

One of his greatest contributions to microbiology was the formulation of the four Postulates that now carry his name. The postulates are step wise, each postulate based on the previous finding.
In order to establish that an organism causes a disease the following requirements have to be fullfilled:

Step 1: Association- The organism and the disease are observed together consistently.

Step 2: Isolation - The organism can be isolated from the diseased.

Step 3: Inoculation - The isolated organism causes the disease in a healthy individuum.

Step 4: Re-isolation - The organism can be re-isolated from the infected individuum.

Now look at each of these steps carefully and think about what they require you to do. Did you notice it? Between each of the steps the principles of the scientific method are applied. Here is a more graphic representation of the application of Koch's Postulates.

By putting clearly defined rules for what defines a disease causing organism (today referred to as a pathogenic organism or just pathogen), Koch made a major contribution to the then raging discussion about the cause and origin of diseases.

Before Kochs discovery of the Cholera bacterium, there was a heated discussion between the Contagionists and the Anticontagionists. The Anticontagionists (Max von Pettenkoffer was one of them, see also this post) argued that in their theory human-to-human transmission was only a very minor component. They were strong opponents of quarantine and disinfection because it inhibited trade and was less effective than local solutions like improved sanitation. You can read more about it here.

Koch's Postulates proved that transmissibility played an important role in epidemics and quarantines and disinfection was indeed a suitable method to counter both. In the end, everyone benefited from the discussion because it improved both local and global safety against infectious diseases. Getting scientists to agree to something is quite difficult but using the right arguments derived from proper use of the scientific method and you have a good chance of succeeding.

Next week I will write about the central dogma of molecular biology and why it is not so dogmatic anymore.

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