Written by MicroDok

Koch’s postulates are sets of scientific criteria which links a particular microorganism to a particular disease (Figure 1). The Koch’s postulates of disease cause are as follows:  

  • The suspected microorganism must be present in every cases of the disease and absent from healthy hosts.
  • The microorganisms must be isolated from the diseased host and grown in pure culture (i.e., a system free from of other types of microbes).
  • The specific disease must be reproduced or seen when a pure culture of the microorganisms is injected into healthy susceptible host.
  • Microorganisms must be recovered again from the experimentally inoculated host and shown to be identical to the original one.

The molecular postulates of Koch’s principle of disease-cause which is mainly interested in the virulence genes present in the pathogenic microorganism (i.e. the genetic information which directs the disease course in the affected host) shall be highlighted in this unit.  

  • The virulence trait under study should be associated much more with pathogenic strains of the species than with non-pathogenic strains.
  • Inactivation of the gene(s) associated with the suspected virulence trait should substantially decrease pathogenicity in the host.
  • Replacement of the mutated gene with the normal wild-type gene should fully develop pathogenicity in the host.
  • The gene should be expressed at some point during the infection and disease process in the host.
  • Therapeutic substances such as antibodies or immune system cells directed against the gene products in the infected host (for healing purposes) should protect the host.

Figure 1: Illustration of Robert Koch’s postulate. Koch hypothesized that a particular microorganism is responsible for causing a particular disease, thus establishing the correlation between a given infectious disease and its causative organism. MicroDok


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