As anyone who’s seen a yogurt commercial knows, our guts are teeming with bacteria. So, too, are our hands, feet, ears, and mouths.
But our brains?
Until recently, scientists would have said no way. The brain was long thought to be a kind of fortress, separated from the body by a virtually impenetrable barrier of specialized cells. Now, that view is beginning to shift, with increasing evidence that aliens can, and do, sneak in.
The latest evidence comes from a team of researchers in Canada, who found that a type of bacteria usually found in soil may make its way into some of our brains.
That possibility is “a mind-bending concept,” said Kathy Spindler, a professor of microbiology and immunology at the University of Michigan who was not involved in the new work. If confirmed, the study would “upset the dogma that the brain is normally a sterile site,” said Vincent Young, an infectious diseases physician and microbiologist also at the University of Michigan. If living bacteria help to maintain brain health in some way, disruptions to them, for example from antibiotics, could contribute to disease. (In other parts of the body, disruptions to native bacteria may play a role in some asthma, food allergies, inflammatory bowel disease, and even obesity, he added.)
The Canadian researchers weren’t looking for bacteria. But in the course of analyzing human brain tissue they came across genetic material typically linked to them. “That’s what tipped us off that there was something going on,” said Christopher Power, a professor of neurology at the University of Alberta who led the research group.
The surprise material turned out to be associated with alpha-proteobacteria, a kind of bug that normally hangs out in soil. The researchers found these bacterial molecules in brain samples from people with HIV, as well as people with no known infectious disease but who had undergone brain surgery. When they ground up human brain tissue and injected it into mice, bacterial molecules were detectable several weeks later in the mice’s brains, suggesting that something of the bugs had stuck around.
Does this mean that living, growing bacteria are crawling around in our white matter? Or that bacterial genetic fragments somehow persisted in the brain? Could be—which is kind of spooky.
“This is the kind of paper that raises fascinating questions,” said Young. A lot of people will now want to prove or disprove the presence of viable bacteria—and figure out what, if anything, the bacteria might be doing.
Power said there was no evidence that the alpha-proteobacteria he found caused disease. Nor was there evidence so far that it provided a benefit, as “good bacteria” do in the gut, for instance, by aiding digestion.
What’s remarkable, though, is that bacteria, or genetic material from bacteria, could be present in the brain at all. That’s because, in order to enter the brain, it would have crossed a boundary often viewed as nearly inviolable. This dogma dates back to the 1800s when researchers first noted that dyes injected into the bodies of animals tended not to show up in their brains, Spindler said. Dyes injected into the brain, meanwhile, tended not to appear in the body. So evolved the concept of a blood-brain barrier: that is, an interface that allows nutrients and certain key molecules to cross into the brain, while keeping most other compounds out. The brain is a castle and this is its moat, as experts have described it.
Yet as recent work suggests, invaders have found all sorts of ways into the inner sanctum. Scientists have discovered, for instance, that HIV hides inside white blood cells that enter the brain in order to look for pathogens; they call this the Trojan horse strategy. (Power speculates that alpha-proteobacteria might enter the brain the same way.) Researchers have also shown that the herpes virus sneaks into the brain by moving along the axons of nerve cells. Other viruses may simply break through blood vessel walls, slipping between or even through the cells, Spindler said. Some foreigners in the brain typically cause disease. Others seem mainly to be quiet, long-term visitors.
And others still are mystery guests whose potential effects are unknown. Last fall, for instance, researchers found male genetic material in the brains of women (who almost certainly were not born with it). Perhaps during pregnancy, the scientists suggested, cells from male fetuses had crossed the placenta and entered the women’s bodies. But how exactly did those fetal cells (or some of the DNA from them) cross the blood-brain barrier and enter the brain? How did they persist for so long, and what, if anything were they doing?
The work on bacteria raises similar questions. It also nudges us toward a different view of the brain—in which a bit more otherness is present, intertwined with self. Maybe deep in our brains, a few bacteria are nestled near some quiescent virus and a touch of fetal DNA? Maybe we really are hybrid creatures through and through.