I know what you’re thinking.
Yeah, yeah, yeah, microbiomics is the new hotness in biology. It’s so easy now to fish out bacterial nucleic acids and sequence them that of course someone has found something suggestive.
But seeing, as the saying goes, is believing.
Electron microscopy has been around for about eighty years. Scientists have always been interested in looking at brains, so you’d expect that if there were any bacteria in brain tissue that someone would have made note of them. Or perhaps not – just as no one thought there could be viruses bigger than bacteria until someone did and started looking for (and finding) them – apparently no one has been looking for bacteria in the brain.
But some folks at the University of Alabama did, reporting their still-tentative findings at the Society for Neuroscience meeting last week. They sound properly skeptical and aware of the possibility of contamination. But now that they are looking, they are finding bacteria in just about every brain. Not just humans, but also mice — except when they look at the brains of germ-free mice. It’s far from case-closed, but it certainly sounds like they are being careful and that there is something here that bears further investigation.
Bacterial brain pathogens – principally Neisseria meningitidis and Streptococcus pneumoniae, have long been known. But these bugs usually infect just the meninges, the triple-layer membrane that encases the brain and spinal chord.
Other bacteria, notably Mycobacterium tuberculosis and Listeria monocytogenes, both intracellular pathogens, are known to infect the brain itself. They likely ride other cells, such as macrophages, into the brain.
But the bugs found in mouse brains are common gut bacteria that rarely or never cause infections. The Alabama group also saw no evidence of inflammation at the sites of bacterial infiltration. There is no reason (for now) to think that they are causing disease. They were just as common in the brains of apparently healthy people as in the brains of schizophrenics.
So what are the bacteria doing there and does it matter? The link between gut microbiota and the brain and behavior is well-established. Many bacteria both produce and respond to neurotransmitters and their precursors, so it’s not much of a stretch to think that they could modulate brain function:
From The Neuro-endocrinological Role of Microbial Glutamate and GABA Signaling
The brain parasite Toxoplasma gondii is thought (not by everyone) to modify human behavior in fairly subtle ways. Might the same be true of gut bacteria in the brain? It seems to me unlikely that they could live there and have no effect at all. Bacteria always manipulate their immediate environments and I see no reason to expect the brain to be different.
The most basic truth in biology is that everything is connected to everything else. Microbiomics leads us to reject the notion of absolute human autonomy with clear demarcations between human and non-human, that we are instead Holobionts whose nature is entwined with that of other creatures. It’s a humbling concept and humility is always a good thing for humans. I hope the Alabama research holds up to further scrutiny.
Update: It appears this report (it was a conference presentation, not a peer-reviewed paper) did not hold up. I searched PubMed in September 2024. All of the authors have numerous publications since this presentation (2018). None of them concern the possibility of a brain microbiome. So it’s likely they were just seeing contaminants and couldn’t repeat their observations.