Koch’s 4 postulates: what are they and what do they explain?

A summary of Robert Koch's postulates, part of the History of Medicine.

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

Robert Koch, together with other scientists, discovered that many diseases had an infectious origin, that is, they were caused by pathogenic agents, such as bacteria.

On this basis, he proposed several statements, known as the Koch's postulateswhich have acquired great importance in the history of microobiology and in the study of infectious diseases. Next we will see why, and what exactly these postulates say.

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. For this reason they are also known as the Koch-Henle model. The postulates were presented in 1890 at the International Congress of Medicine in Berlin for the first time.

These postulates were a major milestone in the history of medicine and contributed to the emergence of microbiology.. Furthermore, it was a turning point in the history of medical sciences, since Koch's proposal has been considered a true bacteriological revolution, allowing us to understand how the relationship between pathogens and diseases. Before this model, many people, doctors and scientists included, believed that diseases could be caused by celestial designs, miasmas or astrology.

In spite of all this, with the passage of time they ended up being revised, proposing updates more adapted to the scientific knowledge of the following century. In addition, the original conception of these four postulates the original conception of these four postulates had certain weaknesses.This made even Koch himself aware that the study of infectious diseases would have to be studied in greater depth.

What are they?

Koch's original postulates were 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 should be able to be found in abundance in all organisms that are suffering from the disease, but 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 abundance in all organisms suffering from that disease, it should be found in all organisms that are suffering from that disease, while healthy individuals should not have it..

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

Asymptomatic persons or those with very mild symptoms are a very common phenomenon in several infectious diseases.. Even Koch himself observed this occurring 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.

2. Second postulate

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

The experimental application of Koch's postulates begins with this second statement, which means that if there is a suspicion that a microbe causes a disease, it should be possible to isolate it from the infected individual. should be isolated from the infected individual and cultured separately, e.g., in an in vitro culture.for example, in an in vitro culture under controlled conditions.

This postulate also stipulates that the pathogenic microorganism does not occur in other infectious contexts, nor does it occur by chance. 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, this postulate fails with respect to viruseswhich, since they are obligate parasites, and taking into account the techniques of the end of the 19th century, it was not possible to extract them for culture under controlled conditions. They need cells in which to be hosted.

3. Third postulate

"The microorganism that has been grown in 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 grown in culture and is present in the appropriate quantity and stage of maturation to cause pathology, it should be able to cause disease when inoculated into a healthy individual, when inoculated into a healthy individual it should cause disease..

When introduced in a healthy individual, the same symptoms should be observed, over time, as those present in the diseased individuals from which the pathogen was extracted.

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

It is now known that the fact that an individual with the pathogen does not show the disease may be due to individual factors, such as being in good physical health, having a healthy immune system, having been exposed to the agent previously 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 experimentally inoculated individuals, and be identical to the pathogen extracted from the first diseased individual from whom it was extracted."

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

Evans revision

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

Evans' postulates are as follows:

• The proportion of diseased individuals should be higher among those who have been exposed to the suspected cause compared to those who have not.
• Exposure to the suspected cause or pathogen should be more frequent among those individuals who have the disease than in those who do not.
• The number of new cases of the pathology should be markedly higher in individuals exposed to the suspected 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 represented in a bell-shaped graph.
• After exposure, the host should exhibit a wide range of responses, from mild to severe, along a logical Biological gradient.
• By prevention or intervention in the host, the symptoms of the disease should be diminished or eliminated.
• Experimental reproduction of the disease should be more frequent in organisms exposed to its presumed cause, as compared to those who have not been exposed. This exposure may be deliberate in volunteers, experimentally induced in the laboratory, or demonstrated by controlled modification of natural exposure.
• Elimination or modification of the presumed pathogenic cause should reduce the frequency of disease presentation.
• Prevention or modification of the host organism's response should reduce or eliminate the disease produced after exposure to the agent.
• All relationships and associations of the pathogen with the disease should be biologically and epidemiologically plausible.

Limitations of the Koch-Henle model

It must be understood that the postulates, although they were an important milestone that accentuated the bacteriological revolution, were conceived in the 19th century.. Considering that science tends to advance 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 did not fit the Koch-Henle model, the postulates have had to be revised, one example being Evans' proposal. Koch's postulates have been considered fundamentally obsolete since the 1950s, although there is no doubt that they are of 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 would seem that the pathogen-disease causal relationship is much more complex than what the model originally proposed, which conceived this causal relationship in a much more linear way than how diseases and their relationship with pathogens are known to occur today.

Bibliographical references

• Byrd, A. L., & Segre, J. A. (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.