It is often forgotten that scientists have theorized that microorganisms play a significant role in many aspects of our lives. Over 100 years ago, Elie Metchnikoff hypothesized that microbes may even play a role in the aging process. Hubert J. Norman and George Porter Phillips tested concepts relating to the benefits of lactic acid bacteria on depression.
More recently, advances in scientific technology have allowed scientists to explore the potential benefits of microorganisms more deeply. It has become clear that there is a connection between the brain and gut bacteria via the vagus nerve and that this relationship has an impact on multiple brain processes, and even on social behavior.
The key is the ability of gut bacteria to create neurotransmitters, such as noradrenaline or dopamine. These chemicals can influence all sorts of conditions including depression and addiction. Let’s take a closer look at the impact of the gut microbiome on stress related disorders such as anxiety and depression.
As more information is gathered about the gut microbiome, many studies have observed that there may be a link between depression and a lack of diversity in microorganisms in the gut microbiome, which is called dysbiosis. It has also been shown that specific strains of bacteria may have an impact in overall well being. For instance, it has been shown in rats that low occurrences of Lactobacillus relates to chronic stress. In this particular study, there was also a decrease in the Firmicutes/Bacteroidetes bacteria ratio. This ratio has also been studied as a factor in patient obesity levels, with mixed results. On a side note, higher levels of Oscillibacter have been seen as a player tied to chronic stress results in dysregulation of the microbiota of rats.
When looking at depressed human patients, scientists are studying their fecal microbes and transplanting these microbes into animals, which is showing that after transplantation the animals then also exhibit depressive behavior. These animals also show an increase in the kynurenine/tryptophan ratio, which may mean an elevated level of the metabolite kynurenine. This means that metabolite production could be an impactful factor in depression.
Scientists have also studied the transfer of microbiota of individuals with irritable bowel syndrome (IBS) to mice with similar types of results; the mice inherited gastrointestinal issues as well as anxiety symptoms. This shows us that the disruption of gut microbiota can result in depression or anxiety-like symptoms.
How do Prebiotics tie into Probiotics and Depression?
In a recent study a prebiotic was given to mice that were subject to stressful situations. The prebiotic helped to improve symptoms of depression in the mice, emphasizing not only the importance of maintaining a healthy gut microbiome, but also the importance of a probiotic colony needing appropriate amounts of food for the bacteria, or prebiotics.
Further, the probiotic strain Bifido longum has been shown to result in self-reported lower levels of stress in healthy male volunteers, making B. longum an excellent candidate for a psychobiotic or mood probiotic. In another study, a fermented milk product was given to female volunteers to test the potential psychobiotic effects. It was shown that the probiotic strains altered the brain regions associated with emotions in the subjects, suggesting a possible link to probiotics having psychobiotic or mood altering effects.
Sherwin E, Dinan TG, Cryan JF. Recent developments in understanding the role of the gut microbiota in brain health and disease. Ann N Y Acad Sci. 2018;1420(1):5-25. doi:10.1111/nyas.13416