Review of microbiome research in brain disease finds intriguing associations in an emerging field

Almost half the cells and just one per cent of the unique genes found in our bodies are human: the rest are from bacteria, archaea, fungi, and viruses. Together, they weigh roughly three pounds, about the same amount as the human brain. Most of those microbes are in our gastro-intestinal tract, with the highest microbial density and activity found in the colon. Together, in their diverse variety and activity across many body sites, these microorganisms comprise the human microbiome.

Gut microbiota contribute to a variety of functions, from facilitating digestion and metabolism to helping regulate and educate the immune system and more recently, also to brain health. The role that the human microbiome plays in disease risk and progression, especially neurodegenerative disease, is only just beginning to be understood by researchers; it is with this in mind that Drs Helen Tremlett, Emmanuelle Waubant (University of California, San Francisco), Silke Cresswell and B. Brett Finlay, with PhD student Kylynda Bauer, have sought to summarize recent discoveries. Their recent publication in the journal Annals of Neurology provides a high-level overview of this rapidly developing field to aid researchers and the neurological community in understanding the complexity of human microbiota and its impacts.

The researchers looked at journal articles published in English over a six-year period, with a focus on neurodegenerative diseases studied in humans.

“Right now, there is exciting and growing evidence to suggest that the microbiota are associated with some diseases of the brain. However, more work is still needed.” says Dr. Tremlett. “One real challenge will be to prove a causative link.” 

Changes to the composition of bacteria in the gut have been found to influence health in a number of ways, possibly including brain health. To understand how this happens, researchers have begun to explore the microbiome in much more detail than ever before – microbiota influence the body in myriad ways, and early discoveries have shown that there is a link between gut bacteria, immunity and brain health.

“Alterations in the gut microbiota have been linked with numerous neurological conditions. This review highlights some of the emerging studies in ‘bug-to-brain’ research,” says Kylynda Bauer, a PhD student in the Finlay Lab. “The mechanisms involved in these gut microbiota-brain interactions are still largely unknown, and future work may provide valuable insights into neurological health. This is an exciting time to be involved in gut microbiota research.”

Challenges still remain, as do opportunities for individuals from many disciplines to be involved in advancing the field. From optimal study design and subject selection through to analyses of ‘big data’ and application of appropriate statistical techniques.

“There is real opportunity to take these discoveries about the human microbiome and incorporate into ongoing research initiatives, and, also into the educational curricula - from schools to Universities” says Dr. Tremlett. “Microbiome research produces a large amount of data, so not only is there a lot of information out there for those interested in the research, there is a wealth of untapped possibilities for those looking to incorporate microbiome studies into existing or prospective neurological research.” 

Citation

Tremlett H, Bauer K, Appel-Cresswell S, Finlay BB, Waubant E. The gut microbiome in human neurological disease: A review. Ann Neurol. 2017 Feb 21. doi: 10.1002/ana.24901.