Koch's postulates are not applicable to
  • A
    T. B.
  • B
    Leprosy
  • C
    Cholera
  • D
    Diphtheria

Solution

The association of specific microorganisms with the disease came about as a consequence of the work of the German physician Robert Koch. He formulated a set of criteria that could be used to identify the pathogen responsible for a specific disease. The Koch's postulates were :

1. The bacteria has to be present in every case of the disease, that is, the evidence of the presence of this bacteria should be present in each and every diseased sample.

2. The bacteria has to be isolated from the affected host while it is being grown/cultured in the lab, and the bacteria must be grown into a pure culture.

3. The specific disease must be re-produced when a pure culture of the bacteria is inoculated into a healthy host. This implies that the isolated and cultured pathogen should be able to cause the same infection in other hosts, even after several generations in culture.

4. The bacteria must be recoverable from the experimentally infected host, that is, the healthy host that it has been inoculated into.

Now out of the four options mentioned in the question, it has been observed that unlike the other three, the causative agent of leprosy, Mycobacterium leprae cannot be cultured in a laboratory. This is because most of the genes in the M. leprae genome are pseudogenes, which means that they have lost their functional status through the course of evolution. These genes are no longer able to code for proteins, which is why the M. leprae are not able to thrive without a host machinery and very specific environmental conditions. These factors make it impossible to obtain a pure culture of the causative agent of leprosy, which shows that it does not obey all the four postulates stated by Koch. 

So, the correct answer is  "Leprosy".