Koch's 4 postulates: what they are and what they explain

There was a time when it was not known what caused disease. There were those who thought they were due to celestial designs, others due to miasmas, and others due to the position of the stars.

Robert Koch, along with other scientists, discovered that many diseases were infectious in origin, that is, they were caused by pathogens such as bacteria.

Based on this, he proposed several statements, called Koch's postulates, which have acquired great importance in the history of microbiology and in the study of infectious diseases. Next we will see why, and what exactly these postulates say.

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What are Koch's postulates?

Koch's postulates are four criteria that were designed to establish the causal relationship between pathogens, mostly microbes, and diseases. They were formulated in 1884 by the German physician Robert Koch, in collaboration with Friedrich Loeffler, based on concepts previously described by Jakob Henle. It is for this reason that they are also known as the Koch-Henle model. The postulates were presented in 1890 at the International Medical Congress in Berlin for the first time.

These postulates have been a great milestone in the history of medicine, and have contributed to microbiology rearing its head. In addition, it marked a before and after in the history of medical sciences, given that Koch's proposal has been considered a true bacteriological revolution, allowing us to understand how the relationship between pathogens and the diseases. Before this model, many people, including doctors and scientists, believed that diseases could be caused by heavenly designs, miasmas, or astrology.

Despite all this, with the passage of time they ended up being revised, proposing updates more adapted to the scientific knowledge of the following century. Besides, the original conception of these four postulates had certain weaknesses, which made even Koch himself aware that he would have to go deeper into the study of infectious diseases.

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Which are?

Koch's original postulates numbered three when they were first presented at the Tenth International Congress of Medicine in Berlin. The fourth was added in later revisions:

1. first postulate

"The microorganism must be able to be found in abundance in all organisms that are suffering from the disease, but it should not be found in those that are healthy."

This means that if a microbe is suspected to be the causative agent of a particular disease, it should be found in all organisms suffering from the disease, whereas healthy individuals should not have it.

Despite the fact that this postulate is fundamental within Koch's bacteriological conception, he himself he abandoned this universalist conception when he saw cases that broke this rule: carriers asymptomatic.

People who are asymptomatic or have very mild symptoms are a very common phenomenon in various infectious diseases.. Even Koch himself observed that this occurred in diseases such as cholera or typhoid fever. It also occurs in diseases of viral origin, such as polio, herpes simplex, human immunodeficiency virus (HIV) and hepatitis C.

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2. second postulate

"The microorganism must be able to be extracted and isolated from a diseased organism and grown in a pure culture."

The experimental application of Koch's postulates begins with this second statement, which goes to say that if a microbe is suspected of causing a disease, this should be able to be isolated from the infected individual and cultured separately, for example, in an in vitro culture with controlled conditions.

This postulate also comes to stipulate that the pathogenic microorganism does not occur in other infectious contexts, nor in a fortuitous way. That is, it is not isolated from patients with other diseases, in which it can be found as a non-pathogenic parasite.

However, this postulate fails with respect to viruses, which, since they are obligate parasites, and taking into account the techniques of the late 19th century, it was not possible to extract them to cultivate them under controlled conditions. They need cells in which to stay.

3. third postulate

"The microorganism that has been grown in a culture should be able to cause disease once introduced into a healthy organism."

That is, according to the Koch-Henle model, if a bacterium has been cultivated in a culture and is present in the appropriate amount and stage of maturation to cause a pathology, when inoculated into a healthy individual it should cause the disease.

When introducing it into a healthy individual, over time, the same symptoms that occur in sick individuals from which the pathogen was extracted should be observed.

This postulate, however, is formulated in such a way that "should" is not synonymous with "should always be". Koch himself observed that in diseases such as tuberculosis or cholera, not all organisms that were exposed to the pathogen would cause the infection.

Today it is known that the fact that an individual with the pathogen does not show the disease may be due to individual factors, such as having a good physical health, a healthy immune system, having previously been exposed to the agent and having developed immunity to it, or simply having been vaccinated.

4. fourth postulate

"The same pathogen should be able to be re-isolated from individuals that were experimentally inoculated, and be identical to the pathogen extracted from the first diseased individual from which it was extracted."

This last postulate was later added to the Berlin Medical Congress at which Koch presented the three previous postulates. It was added by other researchers, who considered it relevant, and basically stipulates that the pathogen that has caused the disease in other individuals should be the same one that has caused the disease in the first cases.

Evans Review

Almost a century later, in 1976, Sir David Gwynne Evans incorporated some updated ideas on epidemiology and immunology into these principles., especially on the immune response of the hosts triggered by the presence of an infectious microorganism.

Evans's postulates are as follows:

• The proportion of sick individuals should be higher among those who have been exposed to the presumed cause, compared to those who are not.
• Exposure to the presumed cause or pathogen should be more frequent among those individuals who have the disease than among those who do not.
• The number of new cases of the pathology should be remarkably higher in individuals exposed to the putative pathogen compared to those not exposed.
• Over time, the disease should follow, after exposure to the causative agent, a period of distribution and incubation, which should be able to be represented in a bell-shaped graph.
• After being exposed, the host should exhibit a wide range of responses, ranging from mild to severe, along a logical biological gradient.
• Through prevention or intervention in the host, the symptoms of the disease must be diminished or eliminated.
• Experimental reproduction of the disease should be more frequent in organisms exposed to its putative cause, compared to those who have not been exposed. This exposure can be deliberate in volunteers, experimentally induced in the laboratory, or demonstrated by controlled modification of natural exposure.
• The elimination or modification of the presumed pathogenic cause should reduce the frequency of presentation of the disease.
• Prevention or modification of the host organism's response should reduce or eliminate disease produced upon exposure to the agent.
• All pathogen-disease relationships and associations should be biologically and epidemiologically plausible.

Limitations of the Koch-Henle model

You have to understand that the postulates, despite the fact that they represented an important milestone that accentuated the bacteriological revolution, were conceived in the 19th century. Taking into account that science usually advances by leaps and bounds, it is not surprising that Koch's postulates have their limitations, some of them already observed in his time.

With the discovery of viruses, which are acellular pathogens and obligate parasites, along with bacteria that do not coupled to the Koch-Henle model, the postulates have had to be revised, being an example of this the proposal of Evans. Koch's postulates They are considered fundamentally obsolete since the 1950s, although there is no doubt that they have great historical importance..

Another limitation is the existence of pathogens that cause different diseases from individual to individual and, also, diseases that occur with the presence of two different pathogens, or even individuals that have the pathogen but will never manifest the disease. In other words, it seems that the pathogen-disease causal relationship is much more complex than what the model initially proposed, which he conceived this causal relationship in a much more linear way than how diseases are known to occur today and their relationship with agents pathogens.

Bibliographic references

• Byrd, A. L., & Segre, J. TO. (2016). Adapting Koch's postulates. Science, 351(6270), 224-226.
• Cohen, J. (2017). The Evolution of Koch's Postulates. In Infectious Diseases (pp. 1-3). Elsevier.
• Evans, a. S. (1976). Causation and disease: the Henle-Koch postulates revisited. The Yale journal of biology and medicine, 49(2), 175.