Koch's Postulates
Definition: Robert Koch postulated that a microorganism can be accepted as the causative agent of an infectious disease only if specific criteria are fulfilled.
Robert koch
The Four Criteria
- Association: The microorganism should be constantly associated with the lesions of the disease.
- Isolation: It should be possible to isolate the organism in pure culture from the lesions of the disease.
- Inoculation: The same disease must result when the isolated microorganism is inoculated into a suitable laboratory animal.
- Re-isolation: It should be possible to re-isolate the organism in pure culture from the lesions produced in the experimental animals.
The Fifth Criterion (Added Later)
An additional fifth criterion was introduced subsequently which states that antibody to the causative organism should be demonstrable in the patient’s serum.
Exceptions to the Postulates
Exceptions
It is observed that it is not always possible to apply these postulates to study all human infectious diseases. Some bacteria do not satisfy one or more of the criteria:
| Organism(s) | Reason for Exception |
|---|---|
| Mycobacterium leprae & Treponema pallidum |
Cannot be grown in vitro (in pure culture). Note: However, they can be maintained in experimental animals. |
| Neisseria gonorrhoeae | There is no animal model to reproduce the disease. Note: However, it can be grown in vitro. |
Note to Self
Koch's postulates are the historical gold standard, but modern microbiology recognizes limitations like unculturable bacteria and lack of animal models!
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