While many people may experience traumatic experiences during their lives, only some will go on to develop post-traumatic stress disorder (PTSD). It is thought that the reason why it manifests itself in some people, but not others, could be down to a whole host of differences, from living conditions, to an individual’s genetic makeup.

Yet as more research picks apart the differences in the number and diversity of microorganisms living in our guts – known as our microbiome – researchers may have found another contributing factor to whether or not a person develops PTSD: the bacteria they carry around with them.

“Our study compared the gut microbiomes of individuals with PTSD to that of people who also experienced significant trauma, but did not develop PTSD,” explained Dr Stefanie Malan-Muller, who led the research. “We identified a combination of three bacteria – Actinobacteria, Lentisphaerae, and Verrucomicrobia – that were different in people with PTSD.”

As humans harbor so much bacteria, outnumbering human cells 10 to one, it is little wonder that scientists have slowly been unraveling the pivotal role they play in our bodies. Not only do the microorganisms help digest and extract nutrients from the food we eat and break down the medicines we take, but it is now well established that they are also major producers of molecules, such as hormones and neurotransmitters, that can influence our brains.

But this relationship is not a one-way street. While the bacteria in our guts have an effect on the brain, the brain can also impact the diversity and composition of microorganisms that survive. Stress can limit bacterial growth, for example, and affect the lining of the intestines, allowing bacteria to enter the bloodstream and cause inflammation, which in turn could influence certain psychiatric disorders. It now seems that the composition of bacteria swimming around inside us could precondition some people towards developing PTSD.

The researchers, who have published their study in the journal Psychosomatic Medicine, found that those who had developed PTSD also had significantly lower levels of the trio of bacteria in their guts, compared to those who had also experienced trauma but had not developed the condition. Interestingly, they also found that experiencing trauma in early life may precondition people to be more likely to get PTSD later in life, by permanently altering their gut flora.

That’s not to say that if you have low levels of these microorganisms you will develop PTSD in response to trauma, or that if you have high levels you never will, simply that the bacteria may be playing a role in relation to immune system regulation and inflammation.