Soldiers and emergency workers could soon be given a jab of friendly bacteria to protect against posttraumatic stress disorder (PTSD), a study found.
The probiotic-based immunisation could also be used to treat anxiety and depression.
Injections of beneficial bacteria can have long-lasting anti-inflammatory effects on the brain making it more resilient to the physical and behavioural effects of stress,
The bacteria Mycobacterium vaccae was first isolated from the mud on the shores of Lake Kyoga in Uganda.
Senior research associate Matthew Frank in the Department of Psychology and Neuroscience at the University of Colorado Boulder said if the findings are replicated in clinical trials it could lead to an anti-stress jab.
He said: “We found that in rodents this particular bacterium, Mycobacterium vaccae, actually shifts the environment in the brain toward an anti-inflammatory state
“If you could do that in people, it could have broad implications for a number of neuroinflammatory diseases.”
Anxiety, PTSD and other stress-related mental disorders affect a quarter of all people at some point in their lives.
PTSD is estimated to affect about one in every three Britons who have a traumatic experience, but it’s not clear exactly why some people develop the condition and others don’t.
Mounting research suggested stress-induced brain inflammation can boost risk of such disorders, in part by impacting mood-influencing neurotransmitters like norepinephrine or dopamine.
Dr Frank said: “There is a robust literature that shows if you induce an inflammatory immune response in people, they quickly show signs of depression and anxiety.
“Just think about how you feel when you get the flu.”
Other studies suggested trauma, illness or surgery can sensitise certain regions of the brain, setting up a hair-trigger inflammatory response to subsequent stressors which can lead to mood disorders and cognitive decline.
Dr Frank said: “We found that Mycobacterium vaccae blocked those sensitising effects of stress too, creating a lasting stress-resilient phenotype in the brain.”
A previous CU Boulder study found mice injected with a heat-killed preparation of M. vaccae and then placed with a larger aggressive male for 19 days exhibited less anxiety-like behaviour and were less likely to suffer colitis or inflammation in their peripheral tissues.
The new study published in the journal Brain, Behavior and Immunity, Dr Frank and senior author associate professor in integrative physiology Christopher Lowry set out to find out what exactly M. vaccae does in the brain.
Male rats injected with the bacterium three times, one week apart, had significantly higher levels of the anti-inflammatory protein interleukin-4 in the hippocampus, a brain region responsible for modulating cognitive function, anxiety and fear, eight days after the final injection.
After exposure to a stressor, the immunised animals also showed lower levels of a stress-induced protein, or alarmin, called HMGB1, believed to play a role in sensitising the brain to inflammation.
They also had a higher expression of CD200R1, a receptor key for keeping glial cells (the brain’s immune cells) in an anti-inflammatory state.
The immunised rats, as in the first study, exhibited less anxious behaviour after stress.
Prof Lowry said: “If you look at the field of probiotics generally, they have been shown to have strong effects in the domains of cognitive function, anxiety and fear.
“This paper helps make sense of that by suggesting that these beneficial microbes, or signals derived from these microbes, somehow make their way to the hippocampus, inducing an anti-inflammatory state.”
He added one day M. vaccae could one day be given to people at high risk of PTSD such as soldiers preparing to be deployed or emergency workers to buffer the effects of stress on the brain and body.
It could also possibly be used to prevent sepsis-induced cognitive impairment.
Prof Lowry is also working with University of Colorado Denver researchers to explore whether veterans with PTSD can benefit from an oral probiotic consisting of a different bacterial strain, Lactobacillus reuteri.
He said: “More research is necessary, but it’s possible that other strains of beneficial bacteria or probiotics may have a similar effect on the brain.”