Is there a cranial microbiome? Can it be harmful?
Over the decades, scientists have dedicated their entire careers to studying microorganisms on and in our bodies. Although the brain has largely been considered a sterile environment, research has shown evidence of microorganisms living harmlessly in the brain.
In what could be considered a serendipitous event, researchers at the University of Alabama in Birmingham (UAB) made unusual observations in slices of brain tissue from cadavers.(1) At the lab of neuroanatomist Rosalinda Roberts, they look for differences in brain tissue between healthy people and people with schizophrenia in the hours after death. One of the scientists in Dr. Roberts’ lab noticed unidentified rod-shaped objects while examining brain tissue slices with an electron microscope. Although she had seen these odd objects before, she dismissed them at the time. Eventually, the researchers utilized the help of a bacteriologist and discovered that these unexpected objects were bacteria. Dr. Roberts’ team checked every brain they had – 34 in total – and found bacteria in every single one, half of them healthy, half from people with schizophrenia.
Further examination revealed that the bacteria were rod shaped and contained a capsule, nucleoid, ribosomes, and vacuoles. The density of the bacteria varied according to the brain region, with abundant bacteria in the substantia nigra, hippocampus, and prefrontal cortex but sparse numbers in the striatum. The bacteria were also found in star-shaped brain cells known as astrocytes that were near the blood-brain barrier.(2) When the genetic material from the bacteria was sequenced, they found that most of the microbes were from three phyla common to the gut: Firmicutes, Proteobacteria, and Bacteroidetes.
Roberts considered whether bacteria from the gut could have leaked from the blood vessels into the brain in the hours between a person’s death and the brain’s removal. To further investigate this, she looked at healthy mouse brains preserved immediately after the mice were killed. She found bacteria in similar locations found in the human brain samples. She then looked at brains of germ-free mice raised in sterile environments and these mouse brains were void of any microbial life. Also important to note is that the researchers found no signs of inflammation or bacterial disease in any of the brains they examined. More work still needs to be done to completely rule out the possibility of contamination, whether from the air or from surgical instruments, but Dr. Roberts plans to run a few more experiments to further substantiate the findings.
If there truly is a “brain microbiome,” a big question that remains is how the organisms get there. Dr. Roberts hypothesizes that they may have crossed from blood vessels, traveling up nerves from the gut, or even come in through the nose.(1) There is currently no evidence to show whether these bacteria are helpful or harmful as the researchers have not determined if there are any major differences in the bacteria between the schizophrenic and healthy brains. Further investigation is needed to determine whether this possible microbiome can have brain-related health outcomes.
Bacterial presence in the brain is a field that has yet to be explored but the fascinating findings from Dr. Roberts’ lab may lead to discoveries that the gut-brain link may be a lot more direct than originally thought.