The community of bacteria that lives in our gut has a lot to tell us. It can give clues to what we eat, the environment we live in, and diseases and disorders we may have. Now, scientists have linked these bacterial species to how we feel. A new research study found an association between women's gut bacteria and their emotions.
Other studies, done in rodents, have linked the composition of their gut community to emotional and social behaviors, like anxiety and willingness to socialize. The new study associated the response of women's brains to images with the type of bacteria in their gut.
The bacteria that live in our guts is as unique as each of us. The types and number of gut bacteria that make up the community of bacteria collectively called our gut microbiome varies with our age, diet, health status, and our environment.
Other research has shown that the gut microbiome of people who eat typical American diets contains an abundance of bacteria from the phyla Firmicutes and Bacteroidetes, followed by bacteria from the phyla Actinobacteria and Verrucomicrobia. These bacteria were present in people free of disease and felt to represent a healthy gut microbiome.
Many gut bacteria can cause disease, especially if numbers of healthy gut bacteria are reduced, such as through antibiotic use. Clostridium difficile, Salmonella, and some strains of Escherichia coli, like E. coli O157:H7, cause particularly severe infections in humans partly because of the potent toxins they secrete.
Researchers haven't thoroughly puzzled out the reasons for the association of gut microbes with different conditions yet. And the links to brain and behavior changes are even less well understood. Tillisch's new research study gave us some of the first insights into that association in humans.
Researchers have already linked a rodents' gut microbiome to brain chemistry changes that increase risk taking, reduce anxiety, and decrease sociability. So, Tillisch wanted to see if the same link between gut bacteria and emotions was active in humans.
The research team analyzed the feces of 40 women to identify the types and number of bacteria present. Then, the researchers had the women do a functional MRI test — a test that measured brain changes like activity in and connections between different parts of the brain — when the women looked at a set of images.
While the MRI tests the researchers ran don't directly measure human emotions, they were used to indicate brain activity in certain brain regions associated with specific emotions. They also looked for differences in the size of various parts of the brain and compared all brain measurements with the predominant types of bacteria in their gut microbiome.
The women were categorized into two groups based on the abundance of their gut bacteria, either Bacteroides or Prevotella. Both organisms are related; they belong to the same bacterial order —
Bacteroidales — but different families. Most women, 33 of them, had a large number of Bacteroides bacteria in their gut. Seven had mostly Prevotella.
Compared to the Prevotella group, the results from the Bacteroides group's MRIs showed thicker gray brain matter in the frontal cortex and insula regions of their brains, regions involved with complex information processing. The size of their hippocampus, involved in memory formation and processing, was also larger.
MRIs of the Prevotella group showed more connections between emotional, attention processing, and sensory parts of the brain. When viewing negative images, the hippocampus in the Prevotella was less active than the Bacteriodes group. They also had higher levels of negative feelings like anxiety, distress, and irritability after looking at negative images than the Bacteroides group.
The study authors say their results support the concept of brain-gut-microbiota interactions in healthy humans, but their results don't explain the interaction. It could be that bacteria in the gut influence the development of the brain and its activity under different situations or that existing differences in the brain affect the type of bacteria that live in the gut.
Tillisch and her team think the more reasonable explanation is that the brain exerts forces through the nervous system, and stress hormones and immune processes under its control that help determine the type of bacteria that populates the gut. We can't know which explanation is correct until scientists perform more detailed studies of the gut-brain link.
And just a final note on funding: The study was partially funded by Danone Research, and some of the researchers were employed by the research group at the time of the experiments. Another author was on the advisory board of the yogurt product company Dannon and its parent company Danone. The remaining authors were not associated with Danone Research, Dannon, or Danone.
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