Does Gut Bacteria Find Its Way To The Brain?

Attention has been drawn to this subject with high resolution microscopes images of bacteria that are penetrating and inhabiting cells of healthy human brains at the annual meeting of the Society for Neuroscience has drawn . University of Alabama researchers preliminary work using tissues from cadavers has found this to be possible, if true this suggests an unexpectedly intimate relationship between the brain and gut microbes.

The brain is a protected environment partially blocked off from the contents of the bloodstream by a network of cells that surround its blood vessels, any viruses or bacteria that do manage to penetrate the blood-brain barrier often cause life threatening inflammation. Research suggests that gut microbes might affect behavior mood, and risk of neurological disease by indirect means; disruption in the balance of gut microbiomes, for example, could increase production of proteins that may cause Parkinson’s disease if it travels up the nerve connecting to the brain.

The team was investigating differences between schizophrenia patients and healthy people via examining brain tissue preserved hours after death. Courtney Walker noticed unidentified rod shaped objects about 5 years ago in images captured with an electron microscope and dismissed them as she was looking for something else, but she was persistent and consulted with UAB colleagues, and a bacteriologist gave her the news this year that they were bacteria. The team has now found bacteria to be somewhere in all 34 brains they have checked, with half being from those with schizophrenia and the other half from healthy brains.

Wondering whether bacteria from the gut could have leaked into the brains between death and removal studies were conducted to investigate looking at healthy mouse brains which were preserved immediately after death to find more bacteria, this was then repeated with germ free mice raised to be devoid of microbial life which were found to be uniformly clean.

Using RNA sequencing most of the bacteria were revealed to be from 3 phyla common to the gut: Bacteroidetes, Firmicutes, and Proteobacteria. It is not known how they ended up in the brain, but it is possible that they crossed from blood vessels, traveling up nerves from the gut, or in through the nose; and not much can be said of whether they are helpful or harmful, however there was no signs of inflammation observed, suggesting they were not causing harm. The team has not yet quantified or systematically compared the healthy and schizophrenic brains; if major differences are found future research could investigate how the proposed brain microbiome could maintain or threaten brain health.

Initial survey of electron micrographs showed that resident bacteria had puzzling preferences, seeming to inhabit star shaped astrocyte cells that interact with and support nerves, clustered in and around the ends of astrocytes in particular that encircle blood vessels at the blood brain barrier, appearing to be more abundant around neurons sheathed in myelin. Rosalinda Roberts is not able to yet explain the preferences but is curious if the bacteria might be attracted to the fat and sugar in these brain cells.

A possible reason for these not being seen in the brain before may be that few researchers subject postmortem brains to electron microscopy, and may disregard or fail to have recognized bacteria in samples as they had originally. It was noted that contamination still needs to be ruled out, and the team will be hunting for such evidence.

If there really is brain microbiome then there is indeed much to investigate. While not entirely surprising that other things can live in the brain, it is still revolutionary. If these gut bacteria are routine, benign presence may play roles in regulating the brain’s immune activity. This could mean a new molecular factory in the brain with its own needs, it will be a long road to prove this, but what an exciting new path of possibilities it is.