Molecular Koch's Postulates|| 4 Points of Molecular Koch's postulate

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. Genes that satisfy molecular Koch's postulates are often called as virulence factors.

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

Additional resource: 9 Exceptions to Koch's postulate

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