I also understood about the myco, URI part.

What I am trying to grasp is why cull all tub mates? Are we assuming that because they are not getting better that it is something worse than myco that has caused the URI and to prevent the bacterial/viral infection from spreading farther, you need to cull? This is what I am assuming and it makes sense.

Maybe in my stage of my colony, culling multiple tubs would be completely devastating to my production. But then it would benefit my colony in the long run.

And now I am just thinking out loud. Let me know if my ramblings are correct.

There is mycoplasma and then there are other contagious bacteria/viruses that is not myco that can present the same symptoms as myco. No cure for any of the above, but to reduce risk, cull and rebuild with healthy stock to keep the miscellaneous bacteria/virus out.