‘That gut feeling’: how stomach bacteria impact brain chemistry and behavior
Experiments with mice have determined that behavior and brain chemistry varies depending on the type of bacteria in the gut, report Stephen Collins at McMaster University and Premysl Bercik at the Farncombe Family Digestive Health Research Institute.
Working with healthy adult mice, the researchers showed that disrupting the normal bacterial content of the gut with antibiotics produced changes in behavior; the mice became either anxious or less cautious. This change was accompanied by an increase in brain-derived neurotrophic factor (BDNF), which has been linked to depression and anxiety.
When oral antibiotics were discontinued, bacteria in the gut returned to normal, “accompanied by restoration of normal behavior and brain chemistry,” Collins said.
The findings are important because several common types of gastrointestinal disease, including irritable bowel syndrome, are frequently associated with anxiety or depression. In addition there has been speculation that some psychiatric disorders, such as late onset autism, may be associated with an abnormal bacterial content in the gut.
Bercik suggested that these results lay the foundation for investigating the therapeutic potential of probiotic bacteria and their products in the treatment of behavioral disorders, particularly those associated with gastrointestinal conditions such as irritable bowel syndrome.
The research appears in the online edition of the journal Gastroenterology.
Gut bacteria and stress
Another recent study with mice has also demonstrated a connection between gut bacteria in the digestive system and stress response.
Researchers at Ohio State University showed that gut bacterial colonies in mice decrease and immune biomarkers increase in response to stress. They ran a series of experiments using an aggressive mouse as a stressor for docile mice.
At the end of the stress experiments, blood samples and material from inside each animal’s intestine were taken from stressed animals along with samples from a control group. The blood samples were analyzed to detect the levels of two immune biomarkers used to gauge stress: a cell-signalling cytokine molecule and a protein called MCP-1 that summons macrophages, or scavenger cells, to the site of an infection.
The intestinal samples were used to determine the relative proportion of at least 30 types of bacteria residing there.
Compared to the control mice, the stressed animals showed two marked differences: the proportion of one important type of bacteria in the gut (Bacteroides) fell by 20 to 25 percent while another type (Clostridium) increased a similar amount. Also, levels of the two biomarkers jumped 10-fold in the stressed mice, compared to controls.
The researchers concluded that exposure to social stressors “significantly affect gut bacterial populations” while increasing circulating cytokines that regulate inflammatory responses.
Ref.: Bailey MT et al., Exposure to a social stressor alters the structure of the intestinal microbiota: implications for stressor-induced immunomodulation, Brain, Behavior, and Immunity, 2011