Vaccines are designed to protect the patient from disease-causing pathogens by imitating an infection and triggering the production of antibodies, thereby building up the body’s natural defense system so that it can fight off similar pathogens in the future if so needed.

Naturally, this type of preventative treatment is geared towards physical ailments. Think: measles, chickenpox, and the flu. But could they ever be used to treat a mental illness – like PTSD?

The answer might just be yes. Scientists are now another step closer to creating a theoretical “stress vaccine” after identifying a fatty acid (or lipid) in the bacteria Mycobacterium vaccae that appears to dial down the fight-or-flight response in mice. The discovery has been published in the journal Psychopharmacology.

“We think there is a special sauce driving the protective effects in this bacterium, and this fat is one of the main ingredients in that special sauce,” Christopher Lowry, senior author and Integrative Physiology Professor, said in a statement.

For the study, Lowry et al. isolated and chemically synthesized 10(Z)-hexadecenoic acid, a lipid found in M. vaccae. The team studied its interactions with immune cells (called macrophages) when those cells were stimulated, finding it performed a lock-like function, binding to the peroxisome proliferator-activated receptor (PPAR) and inhibiting various pathways that trigger inflammation. The researchers found that pre-treating cells with the lipid reduced inflammation when cells were stimulated.

"We knew it worked, but we didn't know why," said Lowry. "This new paper helps clarify that."

It supports a previous study, published in 2016, that found injections of the bacteria before a stressful event can reduce inflammation of the colon and prevent “PTSD-like” symptoms (at least, in mice). The mice injected with the bacteria also appeared less anxious in later stressful situations.

The researchers pin these promising results on the “old friends” hypothesis – a concept that argues our modern, sanitized world has separated us from beneficial (or "good") microbes (including hookworms) found in our environment that help regulate or suppress inflammation. This could help explain increases in allergies and immune disorders as well as mental health conditions like depression. 

"The idea is that as humans have moved away from farms and an agricultural or hunter-gatherer existence into cities, we have lost contact with organisms that served to regulate our immune system and suppress inappropriate inflammation," said Lowry. "That has put us at higher risk for inflammatory disease and stress-related psychiatric disorders."

Lowry and his team hope that results like these will add confidence to the bacteria's potential as a therapeutic. Indeed, he one day hopes to develop a "stress vaccine" that can be administered to first responders, soldiers, and people with high-stress careers.

And M. vaccae may be just the beginning.  

"This is just one strain of one species of one type of bacterium that is found in the soil but there are millions of other strains in soils," said Lowry. "We are just beginning to see the tip of the iceberg in terms of identifying the mechanisms through which they have evolved to keep us healthy. It should inspire awe in all of us."