THE 4 POSTULATES OF KOCH - BIOLOGY - 2025

The Koch 's postulates are rules, guidelines or principles used to test experimental pathogenicity of an organism known or unknown. Before the German bacteriologist Robert Koch presented these principles in 1883, the cause of many infectious diseases was unknown, and many scholars of the subject proposed that they were the product of the "wrath of the gods", of the configuration of the stars. or of "miasmas".

It was not until the early nineteenth century that some scientists accepted that certain microorganisms were the causative agents of most of the common diseases of the time, a fact that was marked, with contributions from different researchers, as a "bacteriological revolution."

Portrait of Robert Koch (Unknown author / Public domain, via Wikimedia Commons)

In this historical context, the demonstration of the microbial origin of tuberculosis and anthrax carried out by Koch was essential for such a “revolution” or paradigm shift, and at that time it represented a great support for the postulates that he himself proposed a short time. after.

Koch's postulates served to give a certain "order" and scientific rigor to the highly controversial debate about infectious diseases and their main causes and, with some exceptions, they have a certain validity today, both in the field of medicine and in medicine. biology.

Among these exceptions are diseases caused by viruses that, with the birth of clinical virology as a discipline, became the focus of attention of many researchers who would later review the postulates and propose new ways of dealing with the issue.

What are Koch's postulates?

According to the presentation made by Robert Koch in 1890, in the framework of the Tenth International Congress of Medicine in Berlin, the postulates are 3:

1- The microorganism that is suspected to cause a disease must be constantly associated with the pathology and in circumstances related to the pathological and clinical changes of the course of the disease

In simpler words, Koch's first postulate states that, if a certain microorganism is suspected to be the causative agent of a particular disease, it should be found in all patients (or organisms) presenting symptoms.

2- The microorganism that is suspected of causing a disease must be able to be isolated from the plant or animal that is sick and must be able to grow in

The experimental application of Koch's postulates begins with the second postulate, according to which, the microorganism that causes a disease must be able to be isolated from the organism it infects and cultivated under controlled conditions.

Image by Anastasia Gepp at www.pixabay.com

This postulate also stipulates that the microorganism in question does not occur in other infectious contexts, nor by fortuitous causes, that is, it is not isolated from patients with other diseases in which it is found as a non-pathogenic parasite.

3- When a healthy susceptible host is inoculated with the grown pathogen

This postulate proposes that pathogenic microorganisms isolated from an infected patient and grown in vitro must have the ability to infect other healthy individuals when they are inoculated with these and, in new individuals, the same clinical symptoms of the patient from where they were isolated should be observed..

In vitro culture of a microorganism in solid medium (Source: Ufficio Comunicazione, Azienda Ospedaliera SS. Antonio e Biagio e Cesare Arrigo, Alessandria and Biblioteca Biomedica Centro di Documentazione / CC BY-SA (https://creativecommons.org/licenses/by -sa / 4.0) via Wikimedia Commons)

4- The same pathogen must be able to be re-isolated from the experimentally infected hosts.

This last postulate was later added by other investigators who considered it relevant and simply stipulates that the facts presented in the last two postulates described should be true when infectious microorganisms are isolated from experimentally infected patients and new patients are inoculated.

In summary, the original postulates, then, state that:

- in an infectious disease the microorganism occurs in each case

- not found in healthy individuals and

- when it is isolated from an infected organism and propagated in vitro, it can be used to infect others and induce the same disease

Some revisions and additions to the postulates

Evans (1976)

In the same way, Evans, in 1976, incorporated into these some epidemiological principles and immunological notions of the host immune response triggered by an infectious microorganism.

Evans's postulates are:

- The prevalence of a disease should be much higher in those hosts that are more exposed to the causative agent than in less exposed controlled cases

- Exposure to the causative agent of a disease should be present much more commonly in those with the disease than in healthy controls, especially when the risk factors remain constant

- The incidence of the disease should be significantly higher in those individuals exposed to the causative agents than those who are not exposed

- Temporarily, the disease should follow exposure to the causative agent and its distribution and incubation periods should be able to be represented in a bell-shaped graph

- After exposure to the causative agent of a disease, a spectrum of responses should occur in the host that follow a biological gradient from "mild" to "severe"

- After exposure to the causative agent, measurable responses should appear regularly in the host

- The experimental reproduction of the disease should occur with a higher incidence in animals or humans exposed to the causative agent than in those that are not exposed; exposure should occur in volunteers, be experimentally induced in a laboratory or demonstrated in a controlled regulation of natural exposure

- Elimination or modification of the putative cause or the transmitting vector should decrease the incidence of the disease

- Prevention or modification of the host response after exposure to the causative agent of the disease should reduce or eliminate the disease

- Everything should make biological and epidemiological sense

Others

Other authors proposed the "molecular postulates of Koch", which are nothing more than an attempt to update the original concepts raised by this microbiologist and which seek to create a kind of "context" that allows the identification of genes associated with the virulence of the microorganism being studied:

- The investigated phenotype should be associated with the pathogenic members of a genus or a pathogenic strain of a certain species

- Specific inactivation of genes associated with the suspected virulence trait should lead to measurable loss of pathogenicity or virulence. Furthermore, these genes should be able to be isolated by molecular methods and their inactivation or deletion should lead to a loss of function in the experimental clone.

- Allelic reversal or replacement of the mutated gene should lead to restoration of pathogenicity. In other words, the restoration of pathogenicity should accompany the reintroduction of wild-type genes.

Limitations of Koch's postulates

Many debates arose after Koch proposed his postulates in the early 1880s. The debates did not question the veracity of the postulates, but rather made it clear that they were applicable to a very limited number of cases.

Thus, with the rapid growth of the field of microbiology, more and more new species of bacteria became known and, shortly after, the participation of viruses in many human diseases.

Among the first conjunctures of his postulates, Koch himself realized that there were apparently healthy patients who were carriers of Vibrio cholerae, as well as other pathogens that cause a great variety of diseases.

Another of the most important limitations has to do with the impossibility of growing some microorganisms under experimental conditions, including many bacteria and viruses (although these cannot be considered microorganisms).

In addition to this and according to the third postulate, not all individuals who are exposed to an infectious agent or pathogen become infected, as this depends, to a large extent, on the previous health conditions of each person, as well as their ability of immune response.

Other aspects to consider are those related to the pathogenicity of a microorganism: some conditions are caused by several pathogens simultaneously and, in the same way, the same pathogen can, in different organisms, cause different pathological conditions.

References

1. Byrd, AL, & Segre, JA (2016). Adapting Koch's postulates. Science, 351 (6270), 224-226.
2. Cohen, J. (2017). The Evolution of Koch's Postulates. In Infectious Diseases (pp. 1-3). Elsevier.
3. Evans, AS (1976). Causation and disease: the Henle-Koch postulates revisited. The Yale journal of biology and medicine, 49 (2), 175.
4. King, LS (1952). Dr. Koch's postulates. Journal of the history of medicine and allied sciences, 350-361.
5. Tabrah, FL (2011). Koch's postulates, carnivorous cows, and tuberculosis today. Hawaii medical journal, 70 (7), 144.