One of the biggest new areas of medical research in the 21st century is the investigation into the holistic effect of the gut microbiome on the human body. We are rapidly discovering that the vast community of bacteria that live in our gut are actually playing roles in everything from Multiple Sclerosis to depression. Two new studies have now found connections between the gut microbiome and posttraumatic stress disorder (PTSD) and Alzheimer's disease.

Research from a team at the University of Wisconsin School of Medicine and Public Health has compared the gut microbiome of 25 human subjects with Alzheimer's disease to 25 cognitively healthy human subjects.

"By using DNA sequencing to take a 'snapshot' of gut bacterial composition, we found that individuals with dementia had decreased microbial richness and diversity in their gut microbiome compared to people without a diagnosis of dementia," says Nicholas Vogt, first author on the study.

"We were able to identify broad taxonomical changes in gut bacterial composition, as well as changes in abundance of a number of bacterial groups, some of which were more abundant in people with dementia due to Alzheimer's disease and some of which were less abundant."

This is the first research to study a whole microbiome snapshot of patients with Alzheimer's disease. It follows on from some compelling research in 2016 that suggested gut bacteria plays a role in the accumulation of amyloid plaques, a buildup of proteins characteristic of a number of diseases including Alzheimer's, diabetes and Huntington's disease.

The second new study is even more fascinating, zooming in on three particular types of bacteria present in the gut microbiome and relating it to the onset of PTSD.

"Our study compared the gut microbiomes of individuals with PTSD to that of people who also experienced significant trauma, but did not develop PTSD (trauma-exposed controls)," says lead researcher Stefanie Malan-Muller.

"We identified a combination of three bacteria (Actinobacteria, Lentisphaerae and Verrucomicrobia) that were different in people with PTSD."

Those subjects with PTSD were found to have significantly lower levels of all three bacteria compared to the control group of subjects who had been exposed to trauma at some point in their lives but had not ultimately developed PTSD. This indicates to the researchers that low levels of these three bacteria could be resulting in heightened levels of inflammation, impacting the brain and contributing to the symptoms of PTSD.

Of course one of the big issues with understanding the role of gut bacteria in disease is trying to evaluate whether these microbiome differences actually contribute to the disease or are simply a consequence of the disease. It's a classic "what came first - the chicken or the egg?" conundrum, and it's one of the great fundamental questions underlying most microbiome research.

Scientists are still confident that research into these gut-brain connections will help us develop future treatments for a variety of diseases despite a current lack of understanding into these broader gut bacteria mechanisms.

"It does, however, bring us one step closer to understanding the factors that might play a role in PTSD," explains Malan-Muller.

"Factors influencing susceptibility and resilience to developing PTSD are not yet fully understood, and identifying and understanding all these contributing factors could in future contribute to better treatments, especially since the microbiome can easily be altered with the use of prebiotics (non-digestible food substances), probiotics (live, beneficial microorganisms), and synbiotics (a combination of probiotics and prebiotics), or dietary interventions."

The gut bacteria/Alzheimer's study was published in the journal Scientific Reports.

The gut bacteria/PTSD study was published in the journal Psychosomatic Medicine.