Koch’s postulates

Koch’s postulates

Ans: Koch’s postulates ([ˈkɔx]) are four criteria designed to establish causative  relationship  between a microbe  and a disease. The postulates were formulated by  Robert Koch and Friedrich Loeffler in 1884, based on earlier concepts described by Jakob Henle, and refined and published by Koch in 1890. Koch applied the postulates to describe the rtiology of cholera and tuberculosis, but they have been controversially generalized to other diseases. These postulates were generated prior to understanding of modern concepts in microbial pathogenesis that cannot be examined using Koch’s postulates, including viruses (which are obligate cellular parasites) or asymptomatic carriers.They have largely been supplanted by other criteria such as Bradford Hill criteria for infectious disease causality in modern public health. Koch’s postulates have been controversially applied to conclude that HIV does not cause AIDS (in support of HIV/AIDS denialism) and that oncoviruses  do not cause cancers.

Four criteria that were established by Robert Koch to identify the causative agent of a particular disease, these include:

  1. The microorganism or other pathogen must be present in all cases of the disease


  1. The pathogen can be isolated from the diseased host and grown in pure culture


  1. The pathogen from the pure culture must cause the disease when inoculated into a healthy, susceptible laboratory animal


  1. The pathogen must be reisolated from the new host and shown to be the same as the originally inoculated pathogen

However, Koch abandoned the universalist requirement of the first postulate altogether when he discovered asymptomatic carriers of cholera and, later, of typhoid fever. Asymptomatic or subclinical infection  carriers are now known to be a common feature of many infectious diseases, especially viruses such as polio, herpes simplex, HIV and hepatitis C. As a specific example, all doctors and virologists agree that polio virus  causes paralysis in just a few infected subjects, and the success of the polio vaccine in preventing disease supports the conviction that the poliovirus is the causative agent.

The second postulate may also be suspended for certain microorganisms or entities that cannot (at the present time) be grown in pure culture, such as prions  responsible for Creutzfeldt–Jakob disease.

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