[j6m8]

Microbes are CRAZY. They're everywhere. Thermal vent-friendly microbes. Space-friendly microbes. Vacuum-resilient, heat-resilient, acid-resilient. Microbe-free-environment-friendly microbes [1]. It seems hard to imagine that a blood-brain barrier could really keep the brain sterile.

We're lucky to live in a scientific era during which a "gut microbiome" is taken for granted (heck, even FDA-approved treatments depend on it! Google FMT, but don't click "images" from your work laptop), but it wasn't so long ago that we felt microbes were unlikely to live endogenously and harmlessly anywhere in the body.

There were also some hypotheses (untested, if memory serves) that COVID-19 influenced olfactory neurons through direct infection. Don't tell the blood-brain barrier, but if I were a bacterium, the nasal palate would be my ingress strategy. Or maybe the gums or gut — one of the cranial nerves, certainly. [edit] I should clarify — covid is viral, not bacterial, but it does show that this is a potential entry vector.

The central nervous system is incredibly complicated, and our symbiotic relationship with microbes is extraordinary. I think it does a disservice to bacteria to suppose they DON'T get involved in an organ :)

[1] https://www.space.com/ryugu-asteroid-sample-earth-life-colon...

[pazimzadeh]

Our defenses are also crazy. For example, the inner mucus layer of the colon is able to keep trillions of bacteria at bay the vast majority of the time (unless there is inflammation or takeover by specific mucus-eating bacteria). Many of the proteins in the secreted mucus layers are still unknown in function, like FCGBP which comprises up to 40% of the protein content of mucus.

The inner of the two Muc2 mucin-dependent mucus layers in colon is devoid of bacteria https://pubmed.ncbi.nlm.nih.gov/18806221/

Bacteria penetrate the normally impenetrable inner colon mucus layer in both murine colitis models and patients with ulcerative colitis https://gut.bmj.com/content/63/2/281

So I think without trillions of bacteria to exclude, in the absence of any other issues excluding bacteria from the brain seems pretty doable.

Many viruses infect neurons, but they are way smaller than bacteria.