https://www.sciencemag.org/news/2018/11/do-gut-bacteria-make-second-home-our-brains

Her lab looks for differences between healthy people and those with schizophrenia by examining slices of brain tissue preserved in the hours after death. About 5 years ago, neuroscientist Courtney Walker, then an undergraduate in Roberts’s lab, became fascinated by unidentified rod-shaped objects that showed up in finely detailed images of these slices, captured with an electron microscope. Roberts had seen the shapes before. “But I just dismissed them, because I was looking for something else,” she says. “I would say ‘Oh, here are those things again.’”

But Walker was persistent, and Roberts started to consult colleagues at UAB. This year, a bacteriologist gave her unexpected news: They were bacteria. Her team has now found bacteria somewhere in every brain they’ve checked—34 in all—about half of them healthy, and half from people with schizophrenia.

Roberts wondered whether bacteria from the gut could have leaked from blood vessels into the brain in the hours between a person’s death and the brain’s removal. So she looked at healthy mouse brains, which were preserved immediately after the mice were killed. More bacteria. Then she looked at the brains of germ-free mice, which are carefully raised to be devoid of microbial life. They were uniformly clean.

RNA sequencing revealed that most of the bacteria were from three phyla common to the gut: Firmicutes, Proteobacteria, and Bacteroidetes. Roberts doesn’t know how these bacteria could have gotten into the brain. They may have crossed from blood vessels, traveling up nerves from the gut, or even come in through the nose. And she can’t say much about whether they’re helpful or harmful. She saw no signs of inflammation to suggest they were causing harm, but hasn’t yet quantified them or systematically compared the schizophrenic and healthy brains. If it turns out that there are major differences, future research could examine how this proposed “brain microbiome” could maintain or threaten the health of the brain.