[bagels]

You can't find out this way. Removing brains exposes it to an outside environment where there are microbes. You can't tell if they arrived before or after you removed the material.

Related recent story about earth microbes colonizing what was hoped to be a pristine sample of astroid captured in space: https://www.space.com/ryugu-asteroid-sample-earth-life-colon...

[flawn]

Maybe the first point you make is also the argument for why our brain might have a microbiome, as our body is not in any way hermetically enclosed, and e.g. amoebas are able to infect our brains just by going through our nose.