Let's not jump to conclusions. That guardian article is about a pre-print study that used frozen brain samples from a single tissue bank, and RNA sequencing was done on a single machine.
There's a lot of controversy around this[1], because contamination is possible and it's known that the blood brain barrier weakens with age. The sample are all from older individuals.
Contamination is possible, but it's hard to explain the results that way. Definitely needs replication, of course.
I don't see the point of the samples all being from older individuals. Even if it exclusively occurs in older individuals (unlikely), it would still mean that the blood-brain barrier is not 100% effective (which we already know to be true).
> When microbes have been found in the human brain, they are associated with active infections or typically linked to a breakdown in the barrier due to diseases such as Alzheimer’s.
I think the question is whether there is a brain microbiome in healthy people, which seems to still be an open question.
I mean, we know about all kinds of microbes that can be in the brains of "unhealthy" people, and we've known that for ages (taxoplasmosis and CJD come to mind).
The association in the article isn't a case of "we didn't find it in healthy people", but "we found it in more in unhealthy people".
From the article: "It found that, while there was a remarkable diversity of species in the control brains, there were often overgrowths of certain bugs in Alzheimer’s brains."
Evidence is scarce and the article is talking about brains with active illnesses. It could be that there isn't any microbiome normally but if the safety mechanisms fail and bacteria colonize the brain, that would cause the illnesses described.
> Evidence is scarce and the article is talking about brains with active illnesses.
They compared them to control brains, which also had microbes.
> It could be that there isn't any microbiome normally but if the safety mechanisms fail and bacteria colonize the brain, that would cause the illnesses described.
That would describe a scenario where the blood-brain barrier isn't 100% effective, no? If it's not 100% effective (which we know to be true), it is also seems unlikely that 100% of the things that cross the blood-brain barrier cause something that we currently consider a disease. Heck, in healthy individuals, latent taxoplasmosis' effects are often so minimial I don't think we'd call it a disease (otherwise 30%+ of the world has the disease).
https://www.theguardian.com/lifeandstyle/2024/dec/01/the-bra...