Is it necessary for a virus to be isolated "outside" of the host organism to fulfill Koch's Postulates?

Question

My understanding is that it is necessary, but if not, then why not?

Answer 1

Hmm, as far as I understand, Koch's postulate applies only to cellular organisms. Viruses, for one, are acellular organisms (not a cell), and some people will argue with you regarding the biologic and abiologic state of viruses.

Personally, I categorize viruses and the diseases they can cause under the "extensions of Koch's postulate"--much like extensions of Mendelian genetics since the "blending hypothesis", because it is observed, does not fall under the normal Mendelian genetic theories.

To answer your question, yes, it is necessary to isolate the free-living virus itself and re-infect a non-infected, healthy cell with the same virus in order to establish the same pathogenesis and disease manifestation. This is so because getting an infected cell with the virus does not do you any good because, primarily, the virions (pre-formed viruses) are inside, and you have to do special isolation techniques to do so. And second, they are virions, which are viruses but not the ones that are outside the cell, so getting virions may or may not work in your favor, if cell-surface attachment is what you are going for. Another thing you have to think about is sustaining the survival of the virus, because some viruses die upon exposure to air for even a little time, making re-infection a challenge (this is why viruses need to be inside a host constantly to promote their survival).

Hope this helps!