What are Some Exceptions to Koch’s Postulates

Koch’s postulates are the criteria that establish a causative relationship between a microbe and a disease. However, there are five exceptions to Koch’s postulates. This article describes those exceptions to Koch’s postulates in detail.

Key Areas Covered

1. What are Koch’s Postulates
     – Definition, Koch’s Postulates
2. What are Some Exceptions to Koch’s Postulates
     – Findings that Oppose Koch’s Postulates

Key Terms: Culturing, Diseases, Koch’s Postulates, Microorganisms, Symptoms

What are Koch’s Postulates

Koch’s postulates refer to the four criteria established by Robert Koch to identify the causative agent for a particular disease. They include:

1. The pathogenic microorganism must be present in all cases of the disease;
2. The pathogenic microorganism can be isolated from the infected host and grown in a pure culture;
3. The pathogenic microorganism grown in the culture must be able to cause the disease once inoculated into a healthy, susceptible, laboratory organism;
4. The pathogenic microorganism re-isolated from the second host organism must show the original characteristics of the inoculated pathogen.

Figure 1: Koch’s Postulates

What are Some Exceptions to Koch’s Postulates 

Koch’s postulates do not fit with some scientific findings. The exceptions to Koch’s postulates are the following five findings that are in contrary to Koch’s postulates.

1. Some microorganisms that cause diseases have never been cultivated under laboratory conditions. In Koch’s postulates, the blood-born pathogenic microorganisms are cultured in artificial media. However, some microorganisms cannot multiply in artificial growth media. For example, leprae can only grow in armadillos, a type of New World placental mammal.
2. Some diseases are caused by several microorganisms. Microorganisms cause distinctive signs and symptoms in the host as in tetanus and diphtheria. However, several microorganisms may elicit the same disease conditions in various situations. For example, kidney inflammation (nephritis) and pneumonia can be caused by several types of pathogens.
3. Some microorganisms cause several diseases. For example, tuberculosis can cause several disease conditions in lungs, internal organs, bones, skin, etc. S. pyogenescan cause sore throat, erysipelas, scarlet fever, and osteomyelitis. The symptoms of M. tuberculosis are shown in figure 2.

Figure 2: M. tuberculosis Symptoms

4. Some diseases have variable signs and symptoms among patients. For example, tetanus shows variable disease conditions among different individuals.
5. Ethical considerations prohibit the experiments when humans are the only host for a particular disease. If there is an investigation/research on HIV that infects humans, it is unethical to purposely infect a human with HIV.

Conclusion

Koch’s postulates state the relationship between a disease-causing microbe and its disease. However, some of the later findings on disease-causing pathogens are contrary to Koch’s postulates. They are the five exceptions to Koch’s Postulates.

Reference:

1.“Koch’s Bacillus Medical Definition.” Merriam-Webster, Available here.
2.“The Exceptions to Robert Koch’s Postulates.” Health Guide Info, 27 Dec. 2008, Available here.

Image Courtesy:

1. “Koch’s Postulates” By [mike jones] – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Tuberculosis symptoms” By Häggström, Mikael (2014). “Medical gallery of Mikael Häggström 2014”. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.008. ISSN 2002-4436 (Public Domain) via Commons Wikimedia