By Emma Young

After a traumatic experience, why do some people develop Post-Traumatic Stress Disorder (PTSD), while others don’t? Work to date has found evidence that various factors play a role, including a lack of social support and low levels of the neurotransmitter neuropeptide Y (due to its role in the body’s stress response). Into this mix come new findings, reported in Psychosomatic Medicine, that an individual’s complement of gut bacteria (their gut microbiome) may contribute to their vulnerability to trauma. The researchers are now investigating whether tweaking the gut microbiome could help to prevent or treat PTSD.

Lead author on the paper, Sian Hemmings, is based at Stellenbosch University in South Africa, where about three quarters of the population experience at least one traumatic event in their lifetime, and more than half experience multiple traumatic events. PTSD is a debilitating anxiety disorder, which can persist for years. While estimates of its prevalence in South Africa, as well as other countries, vary widely, in the US, for example, about 6.8 per cent of adults are diagnosed with PTSD at some point in their lives. There’s a pressing need for new insights into the factors that contribute to PTSD, and new strategies for prevention and treatment.

Clues to the involvement of the gut microbiome in PTSD come from its role in the body’s response to infection and physical injury (the inflammatory response), which can be affected by stress. Regulatory T cells (Tregs) play an important role in the normal regulation of inflammation, and lowered levels of Tregs have been found in people after they are exposed to stress in the psych lab, and in refugees with chronic PTSD, compared with controls. Since the human microbiome can influence levels of Tregs, the researchers wondered if there might be any differences in the gut microbiomes of people with and without PTSD.

The researchers analysed the gut bacteria (via their faecal samples) of 30 South Africans who had experienced a traumatic event. Eighteen of them had subsequently developed PTSD and they had lower levels of three phyla of bacteria – Actinobacteria, Lentisphaerae and Verrucomicrobia – compared with those who hadn’t developed PTSD. Actinobacteria and Verrucomicrobia include many microbes that have been found to have immunoregulatory properties, such as mycobacteria, and Akkermansia muciniphila, and lower levels may be a sign of an exaggerated inflammatory response.

Christopher Lowry at the University of Colorado, Boulder, a senior author on the new paper, told us he and his colleagues believe that low levels of these bacteria contribute to risk of developing PTSD, although the new findings don’t prove that link.

If future research is consistent with the gut microbiome influencing vulnerability to PTSD, the potential mechanisms for now are unclear. “We currently do not have a good understanding of why elevated inflammation, at the time of trauma, increases risk of PTSD,” Lowry said. “This may involve priming of microglia [a type of cell] in the brain, so that exposure to trauma induces an exaggerated neuroinflammatory response.”

But this study, along with other work linking heightened inflammation to PTSD, does suggest that it’s worth exploring whether raising levels of these bacteria might reduce the risk of the disorder, or even help in treatment. Lowry and his colleagues are in the process of doing just that. In collaboration with US Veterans Affairs scientists, they are currently conducting a clinical trial evaluating the effects of a probiotic (which stimulates the growth of desirable bacteria) on the stress response in veterans with PTSD.

—The Microbiome in Posttraumatic Stress Disorder and Trauma-Exposed Controls: An Exploratory Study

Emma Young (@EmmaELYoung) is Staff Writer at BPS Research Digest