This was a banner week, in terms of incredibly interesting bits of research popping up everywhere. It was hard to figure out what to write about for today! I have ended up choosing a study out of Duke University looking at the effects of mild stress on the bacterial microbiome[i] – a topic in which I’m especially interested. While this research was done on mice, it is extremely relevant to humans – as I will describe.
The paper starts by pointing out the staggering number of people worldwide – 16%!!! – suffering from major depressive disorders. Currently, no one knows the cause of this epidemic. Many studies in humans and animals (some of which I have written about, like here and here) have shown shifts in the microbiota: “This finding has been recapitulated in various animal models as exposure to social stressors known to elicit anxiety and depressive-like behavior have been shown to produce alterations in the structure of the gut microbial communities.”
Bearing in mind, that the gut-brain axis is bidirectional, the researchers wanted to build upon previous research, looking at the very specific changes in gut bacteria that occur due to stress: “Exposure to mild social defeat stress [for 7 days] is sufficient to induce anxiety- and depressive-like behaviors.” Accompanying these behavioral changes (in mice, stress and depression apparently show themselves in behaviors like eating habits, exploratory behaviors, increased “corner time,” etc.), they found “…pronounced decrease in the richness of the microbial community…”
(Don’t you just wish this were available as a probiotic already?!)
A couple more items of interest to share with you:
The researchers also note that the stressed mice also showed increases in pathways related to a variety of metabolic processes, including those involved in Parkinson’s disease.
Also, prior to the experiment, the mice used were classified as “susceptible” and “resilient” (to stress) based upon commonly accepted behavioral guidelines. While both groups showed similar levels of anxiety and depression after the experiment was complete, the scientists did “…observe some microbial differences unique to susceptible mice. In particular, susceptible mice displayed significant increases in Prevotella spp. …This finding echoes recent clinical reports detailing alterations in the proportion of Prevotella spp. within fecal microbial communities of patients with MDD [major depressive disorder] that were consistent with self-reported depression severity.” Going way back to a post I wrote in July 2017 (underlining added for emphasis):
“Researchers at the University of California took fecal samples from 40 healthy women between 18 and 55 years old. They noted that the microbiota composition fell into 2 distinct groupings, with one have more Bacteroides species and the other having more Prevotella species. The women were giving MRIs to examine their emotional responses while looking at pictures designed to provoke positive, negative or neutral emotions.… The women with more Prevotella had a smaller frontal cortex and hippocampus, and showed greater connections between emotional, attentional and sensory regions of the brain. This latter group reported “greater levels of anxiety, distress and irritability when looking at negative images.”
So this finding appears to be consistent for both animals and humans. Interesting, huh?!
Thus, the authors suggest, the fecal microbiota are a potential tool in diagnosing MDD in patients, and also, in tracking their symptoms. Of course, as I am all about “things you can do now”…well, not to make a bad pun but…I am anxiously awaiting the psychobiotic treatment for an illness that adversely affects so many millions of people’s lives.
[i] McGaughey, KD, et. al. Relative abundance of Akkermansia spp. and other bacterial phylotypes correlates with anxiety- and depressive-like behavior following social defeat in mice. Scientific Reports. 2019;9(3281). https://doi.org/10.1038/s41598-019-40140-5