What are Koch's postulates?

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Koch’s postulates are a set of experimental guidelines used to determine if a particular microorganism is the causative agent of a particular disease.
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Koch’s postulates are a set of experimental guidelines used to determine if a particular microorganism is the causative agent of a particular disease.

Historical Overview

In the nineteenth century, Robert Koch, a German physician and bacteriologist, played a significant role in determining the etiology (cause) of an infectious disease. Through his work with Bacillus anthracis (the causative agent of anthrax), he linked a specific microorganism to a specific infectious disease. Koch conducted experiments showing that B. anthracis was always present in diseased animals, that healthy animals inoculated with the bacterium would develop the disease, and that cultivation of the bacterium in artificial media followed by inoculation resulted in the disease.

Koch also discovered the causative organisms for several other diseases, including tuberculosis and cholera. In describing the etiology of tuberculosis, Koch proposed a set of guidelines for establishing a cause and effect relationship between a given microorganism and a specific disease. These scientific criteria are known as Koch’s postulates.

The Postulates

Koch’s postulates are a set of four experimental criteria used to establish the etiology of a disease. The first criterion states that the pathogen must be present in all infected persons and absent in all healthy persons. The second criterion states that the pathogen must be isolated from the diseased person and cultivated in the laboratory. The third criterion states that the cultivated pathogen must cause the disease in a healthy person after inoculation. The fourth criterion states that the pathogen must be isolated again from the infected person and identified as identical to the original isolate.


There are some exceptions to Koch’s postulates. Certain pathogens and fastidious microorganisms have complex and unusual growth requirements and can survive only within living host cells. Such microorganisms cannot be cultured on artificial media. Numerous pathogens infect a specific species only while others become transformed in vitro. Some infectious diseases have unclear origins while others cause multiple disease conditions. Many infections develop from the combined effects of several different microorganisms. Various diseases do not originate from a microorganism and may be the result of poor nutrition, chromosomal abnormality, organ failure, or environmental influences. These exceptions have stimulated the need for modifications to Koch’s postulates.


Koch’s contributions were invaluable in the advancement of medical microbiology and in the understanding of the nature of a disease. Koch’s postulates still provide the essential principles for determining the causative agents of emerging infectious diseases and the basic foundation within which to address disease and public health.


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