Post-traumatic stress disorder (PTSD) doesn’t always develop after a person experiences a traumatic event. Researchers at Stellenbosch University discover new evidence of the important role in PTSD of microbes in the human gastrointestinal tract, called the gut microbiome. These microbes fight infections and metabolize food and medicine, but may also influence the brain and brain function. In this recent study, the researchers compared the gut microbiomes of individuals with PTSD to that of people who also experienced significant trauma, but did not develop PTSD (trauma-exposed control group). They identified a group of bacteria–Actinobacteria, Lentisphaerae and Verrucomicrobia–that were different in people with PTSD. Individuals with PTSD had significantly lower levels of this trio of bacteria compared to trauma-exposed control groups. These bacteria are engaged in immune system regulation, and previous research has revealed increased levels of inflammation and altered immune regulation in individuals with PTSD. These changes in immune regulation and increased inflammation also impact the brain and behavior. Levels of inflammatory markers measured in individuals shortly after a traumatic event, were shown to predict later development of PTSD. These changes in the gut microbiome might play an important role in that higher risk of developing PTSD later in life.