Researchers Found Bacteria in the Human Brain

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There’s a growing awareness that what goes on in our gut somehow affects the function of our brain, though exactly how is still being debated. The brain is thought to be a sterile place, where a barrier protects it from molecular invaders and infection. If the bacteria in our stomach impact the brain, it was thought to be done indirectly—through the chemicals the microbes produce or through nerve connections from the gut to the brain.

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But in a presentation last week at the annual meeting of the Society for Neuroscience, one poster challenged this assumption. “This is the hit of the week,” UCLA neuroscientist Ronald McGregor told Science. “This is mind-blowing.”

The newly presented research from neuroscientist Courtney Walker and neuroanatomist Rosalinda Roberts of The University of Alabama in Birmingham looked at samples of 34 postmortem human brains and found something surprising: bacteria. In every single sample. The brain tissue also didn’t show any signs of disease or inflammation, so it didn’t appear that the bacteria were harming the brain tissues in any way. Most of the bacteria found were also types of microbes found in the gut, according to Science. The bacteria were rod shaped, the abstract of the poster says, and more densely found in certain parts of the brain, including the hippocampus and prefrontal cortex.

Postmortem brains are rarely looked at using electron microscopy, a high-powered imaging technique, which might be why other neuroscientists haven’t noticed bacteria in brains before. “Pairing up a neuroanatomist with a brain collection just doesn’t happen very often,” Roberts told Science.

To rule out the possibility that the bacteria contaminated the brain samples later on, the researchers did follow-up imaging in mice brains. In 10 mouse brains that were fixed immediately after death, they still found bacteria in similar locations. And in mice that were bred to be microbe-free, they didn’t see any bacteria in their brains.

Still, this is early work that needs to be replicated before the presence of a “brain microbiome” is confirmed. But if there are bacteria inhabiting are brains, the next question to follow will be: What exactly are they doing there?

“The brain has always been thought of as a sterile site," Amesh Adalja, an infectious disease physician at Johns Hopkins Center for Health Security told Live Science. "To find [bacteria] there doing no harm sort of breaks a lot of the dogma.”

In 1998, neuroscientist V.S. Ramachandran wrote about a patient named Arthur who, after a terrible car accident, fell into a coma for three weeks. When Arthur woke up, he seemed to recover normally except for one problem: He thought his parents were imposters, fakers. Arthur told Ramachandran that his mother and father still looked like his parents, but something wasn’t quite right.

“Yes he looks exactly like my father but he really isn’t,” Arthur said, as described in Ramachandran’s book Phantoms in the Brain. “He’s a nice guy, doctor, but he certainly isn’t my father!”

Arthur was experiencing Capgras syndrome, which is when a person has the delusional belief that someone—usually a family member or partner—has been replaced by an identical looking imposter. Capgras syndrome can teach us a lot about how a healthy brain recognizes faces, but because it’s so rare, it can be hard to study; it’s usually reported in single case studies, like Arthur’s.

In a study preprint published on PsyArXiv, researchers from University College London and King’s College London continued work from 2017 that scoured medical records to find larger sample sizes of patients with Capgras syndrome to better understand it.

The leading theory on how Capgras syndrome works is that people can consciously recognize familiar faces, but don’t have the appropriate emotional response, “which may be the basis of how the ‘familiar people seeming to be imposters’ delusion arises,” says Emily Currell, a research assistant, cognitive neuropsychiatrist, and co-author on the paper, along with neuropsychologist Vaughn Bell. (This is the opposite problem as face blindness, or prosopagnosia, where it’s thought that people retain the proper emotional response to faces, but aren’t able to recognize or distinguish between them.)

The researchers analyzed medical records of 350,000 people and identified 118 cases of Capgras syndrome. What they found challenges that Capgras syndrome hypothesis. They that a small but meaningful group could experience Capgras syndrome even with people they didn't have that kind of emotional connection to. “Although most affected people do believe a family member or partner has been replaced—in line with the existing theory—about 20 percent have Capgras delusion for people who aren’t well known, meaning either that the current theory is wrong or, more likely, this is a different sort of misidentification delusion that has yet to be explained,” Currell tells me.

More study will need to be done to try and explain what is going on in that 20 percent of patients. But even though they’re a subset of an already rare disease, learning about the brain when something’s gone wrong can be an insightful window into its normal functioning. “Rare disorders reveal the function of the mind and brain by showing the limits and possibilities of what can happen when these systems stop working,” Currell says.

When people lose the ability to see it can come with a strange side effect: hallucinations. It’s called Charles Bonnet Syndrome, and the hallucinations can range from simpler geometric hallucinations to more complex hallucinations of faces, objects or entire scenes, says David Painter a postdoctoral research fellow at The University of Queensland. These hallucinations are not explained by mental illness, medication, or other delusions.

One of the most common reasons for losing vision are eye diseases like macular degeneration (MD), when the retina degenerates with old age. People with MD lose their central vision and have a hard time reading or recognize faces, but can still see in their outer fields of vision.

There’s been a hypothesis about what causes people with vision loss to hallucinate, and it’s that the visual part of the brain—deprived of its normal input from the retina—becomes hyper-excitable. Previous work has shown that when people with Charles Bonnet hallucinate, there is also increased activity in the brain, but those studies might have just been picking up on the hallucinations themselves, Painter tells me.

In a new study in Current Biology, Painter and his colleagues looked to see if people with Charles Bonnet Syndrome had overactive responses to the intact parts of their visual field, even when they weren’t hallucinating.

They brought people into the lab and had them watch a computer screen which they recorded their brains using EEG. They showed them “psychedelic art like patterns of colored checkerboards,” and watched how the brains responded as the checkerboards flickered.

They saw that there was a much stronger response in the visual lobes of the brains of people with Charles Bonnet Syndrome when compared to controls, and also to those with MD but no hallucinations. Their results give support to the idea that this hyper-excitability in the brain is related to the presence of hallucinations, and could lead to treatments to reduce these hallucinations, perhaps through non-invasive brain stimulation. But Painter says that most people with Charles Bonnet Syndrome have a neutral or even positive emotional response to their hallucinations.

In this study, some of the hallucinations people reported seeing were: "The head of a brown lion," wrote the British Psychological Digest. "Multiple tiny, green, spinning Catherine wheels with red edges. Colourful fragments of artillery soldiers and figures in uniform and action. Unfamiliar faces of well-groomed men."

“Individuals who find their hallucinations distressing should be reassured that their hallucinations are benign and not a result of brain disease or mental illness," Painter tells me. "Rather they are natural response to the visual areas of the brain adjusting to changing sensory input.”

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