Koch's Postulates vs Molecular Koch's Postulates

Koch's postulates are a set of criteria used to establish a causal relationship between a microorganism and a disease.

Koch's Postulates

Postulate 1. The microorganism must be present in every case of the disease but absent from healthy organisms.

Koch's Experiment:

  • Koch developed a staining technique to examine human tissue.
  • Mycobacterium tuberculosis cells could be identified in diseased tissue.

Postulate 2. The microorganism must be isolated from the diseased host and grown in pure culture.

Koch's Experiment:

Koch grew M. tuberculosis in pure culture on coagulated blood serum.

Postulate 3. The same disease must result when the isolated microorganism is inoculated into a healthy host.

Koch's Experiment:

  • Koch injected cells from the pure culture of M.tuberculosis into guinea pigs.
  • The guinea pigs subsequently died of tuberculosis

Postulate 4. The microorganism must be re-isolated from the inoculated, diseased individual and matched to the original microorganism.

Koch's Experiment:

Koch isolated M. tuberculosis from the dead guinea pigs and was able to again culture the microbe in pure culture on coagulated blood serum.

Understand More: 9 Exceptions to Koch’s postulate

Molecular Koch's Postulates

In 1988, Stanley Falkow proposed a revised form of Koch’s postulates known as molecular Koch’s postulates using genetic tools. He revised Koch’s postulate incorporating the advancement in the field of microbiology, molecular biology, recombinant DNA technology etc.

Molecular Koch's postulates are used to determine what genes contribute to a pathogen's ability to cause disease.

Molecular Koch's Postulates

Postulate 1. The phenotype (sign or symptom of disease) should be associated only with pathogenic strains of a species.

Falkow’s modifications to Koch’s original postulates explain not only infections caused by intracellular pathogens such as viruses but also the existence of pathogenic strains of organisms that are usually non-pathogenic.


Escherichia coli is a member of the normal microbiota of the human intestine and is generally considered harmless.

Pathogenic strains enterohaemorrhagic E. coli (EHEC): exist because of the acquisition of new genes by the once-harmless E. coli, is now capable of producing toxins and causing illness. 

Postulate 2. Inactivation of the suspected gene(s) associated with pathogenicity should result in a measurable loss of pathogenicity


Pathogenic strains enterohaemorrhagic E. coli (EHEC): One of the genes in EHEC encodes for Shiga toxin, a bacterial toxin (poison) that inhibits protein synthesis. Inactivating this gene reduces the bacteria’s ability to cause disease

Postulate 3. Reversion of the inactive gene should restore the disease phenotype


Pathogenic strains enterohaemorrhagic E. coli (EHEC): By adding the gene that encodes the toxin back into the genome (e.g., with a phage or plasmid), EHEC’s ability to cause disease is restored.

Limitation of Molecular Koch's postulates

Genetic manipulation of some pathogens is not possible using current methods of molecular genetics

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