There was a time when we didn’t know what caused illnesses. There were those who thought they were by heavenly designs, others by miasmas, and others by the position of the stars.
Robert Koch, along with other scientists, discovered that many diseases have an infectious origin, that is, they are caused by pathogens, such as bacteria.
On this basis, he proposed several statements, called Koch’s postulates, Which have acquired great importance in the history of microobiology and in the study of infectious diseases. Below we will see why and what exactly these postulates say.
What are Koch’s postulates?
Koch’s postulates are four criteria designed to establish the causal relationship between pathogens, primarily microbes, and disease. They were formulated in 1884 by the German physician Robert Koch, in collaboration with Friedrich Loeffler, on the basis of the concepts described above by Jakob Henle. It is for this reason that they are also known as the Koch-Henle model. The postulates were first presented in 1890 at the International Congress of Medicine in Berlin.
these postulates were a major milestone in the history of medicine and contributed to the development of microbiology. Moreover, it was a before and after in the history of medical science, since Koch’s proposal was considered a true bacteriological revolution, allowing us to understand how the relationship between pathogens and diseases. Before this model, many people, including doctors and scientists, believed that illnesses could be caused by celestial designs, miasma, or astrology.
Despite all this, over time they ended up being revised, offering updates more suited to the scientific knowledge of the next century. Outraged, the original design of these 4 postulates had certain weaknessesThis made even Koch himself aware that he should deepen the study of infectious diseases.
What are they?
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 should be able to be found in abundance in all organisms which suffer from the disease, but it should not be found in those which are healthy.”
This means that if a microbe is suspected to be the causative agent of a particular disease, should be able to be found in all organisms that suffer from this disease, while healthy individuals should not have.
Although this postulate is fundamental in Koch’s bacteriological conception, he himself abandoned this universalist conception when he saw cases that broke this rule: asymptomatic carriers.
People who are asymptomatic or have very mild symptoms are very common in various infectious diseases.. Even Koch himself observed that this occurs 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 cultivated in a pure culture.”
The experimental application of Koch’s postulates begins with this second statement, namely that if it is suspected that a microbe is the cause of a disease, this it must be capable of being isolated from the infected individual and cultured separately, For example, in in vitro culture with controlled conditions.
This postulate also comes to stipulate that the pathogenic microorganism does not appear in other infectious contexts, nor 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 regard to viruses, Which, as they are obligate parasites, and given the techniques of the late 19th century, it was not possible to extract them for cultivation under controlled conditions. They need cells to house them.
3. Third postulate
“The microorganism which has been cultivated in a culture should be capable of causing disease when introduced into a healthy organism.”
That is, according to the Koch-Henle model, if a bacterium has been grown in a culture and is present in the appropriate amount and stage of ripening to cause pathology, being inoculated into a healthy individual should cause illness.
When introduced into a healthy individual, the same symptoms that occur in sick individuals from which the pathogen has been extracted should be observed over time.
This assumption, however, is worded 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 exposed to the pathogen would cause infection.
It is now known that the fact that an individual with the pathogen does not have the disease may be due to factors of the individual, such as good physical health, a healthy immune system, having previously been exposed to the disease. agent and having developed immunity. of him or simply having been vaccinated.
4. Fourth postulate
“The same pathogen should be able to be isolated again from individuals that have been experimentally inoculated, and be identical to the pathogen extracted from the first diseased individual to which it was extracted.”
This last postulate was later added to the Berlin Medical Congress in which Koch presented the previous three postulates. It was added by other researchers, who found it relevant, and it basically states that the pathogen that caused disease in other people must be the same as the one it caused in the first few. case.
Almost a century later, in 1976, Sir David Gwynne Evans incorporated some up-to-date ideas on epidemiology and immunology into these principles., Especially on the immune response of the drawn hosts in the presence of an infectious microorganism.
Evans’ postulates are as follows:
- The proportion of sick individuals is expected to be higher among those who have been exposed to the alleged cause, compared to those who are not.
- Exposure to the alleged cause or pathogen is expected to be more common in people with the disease than in those without.
- The number of new cases of the pathology is expected to be remarkably higher in individuals exposed to the pathogenic hypothesis 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, from mild to severe, along a logical biological gradient.
- By preventing or intervening in the host, it should decrease or eliminate the symptoms of the disease.
- Experimental reproduction of the disease is expected to be more frequent in organisms exposed to its putative cause, compared to those which have not been exposed. This exposure can be deliberate in volunteers, induced experimentally in the laboratory, or demonstrated by a controlled modification of natural exposure.
- Removal or modification of the putative pathogenic cause should reduce the frequency of disease presentation.
- Prevention or modification of the response of the host organism should reduce or eliminate the disease produced after exposure to the agent.
- All the pathogen’s relationships and associations with disease should be biologically and epidemiologically plausible.
Limitations of the Koch-Henle model
It should be understood that the postulates, although they constituted an important stage which accentuated the bacteriological revolution, were conceived in the XIXe century.. Since science generally progresses by leaps and bounds, it is not surprising that Koch’s postulates had their limitations, some of which were already observed in his time.
With the discovery of viruses, which are obligate cellular pathogens and parasites, as well as bacteria that did not mate with the Koch-Henle model, the postulates had to be revised, being an example of Evans’ proposal. . Koch’s postulates they 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 who have the pathogen but will never manifest. disease. In other words, it seems that the pathogen-disease causal relationship is much more complex than what the model originally proposed, which conceived this causal relationship in a way much more linear to how diseases are known today and to their relationship to pathogens.
- Byrd, AL and Segre, JA (2016). Adaptation of Koch’s postulates. Science, 351 (6270), 224-226.
- Cohen, J. (2017). The evolution of Koch’s postulates. In infectious diseases (pages 1-3). Elsevier.
- Evans, AS (1976). Cause and disease: the Henle-Koch postulates revisited. The Yale Journal of Biology and Medicine, 49 (2), 175.