No one expected that gut bacteria could change mood, but a number of studies have documented this interesting effect. In 2011 a group of scientists in Ireland and Canada developed an interesting model for studying the effects of gut bacteria on the state of mental health in mice. The animals were subjected to a stress test, in which they were forced to swim in an environment in which they had no escape, not something mice generally enjoy doing. The scientists measured what they called ‘‘behavioral despair.’’ This same test had been used to test antidepressant drugs.
What the scientists found was that mice who had been fed for several weeks before with a broth, which contained the bacteria Lactobacillus rhamnosus, a common bacterium that is found in humans and also used to ferment milk into probiotic yogurt, they were able to swim longer and showed less behavioral despair. This led the researchers to hypothesize that the bacteria were somehow altering the chemical environment of the brain, like a drug might do. One hypothesis is that the bacteria somehow affect a nerve in the gut, called the vagus nerve, and that it then sends impulses to the brain, which affect the emotional state of the animal.
Further studies showed that when mice were fed another bacteria, which can cause food poisoning in humans but not mice, the mice showed signs of anxiety. Before having the bacteria they could walk on walkways but after taking the bacteria they hesitated and appeared to be anxious.
Using germ-free mice give more insights into these questions. Germ-free mice tend to have an exaggerated stress response and a reduced anxiety response. When certain bacteria are introduced into these animals the stress response is reduced and anxiety response increased. These gut bacteria had the effect of normalizing the mental health of the animals. Studies on these same animals also reveals that gut microbiota is required for normal social behavior in mice as well as normal repetitive and locomotive behavior like self-grooming. In addition, gut bacteria are important for special and object memory but not for smell memory. The reasons for these changes in mood may have something to do with the levels of certain neurotransmitter, such as serotonin and its precursor tryptophan. Research suggests that the gut bacteria might affect serotonin activity in the brain by changing tryptophan availability in the blood. Research on how gut bacteria affect mood is still at an early stage.
In one brain imaging study, subjects were first given either a psychobiotic mixture or a placebo. Psychobiotic is a new scientific term to describe a probiotic that is used for mental health. They were then shown images of frightened faces. The subjects taking the placebo showed a stress response in which some areas of the brain responsible for emotions became activated. Subjects receiving the psychobiotics showed less activity in these same areas of the brain, which suggests less vulnerability to stress.
These and other studies indicate that changing our gut bacteria really can affect emotional behavior, as well as memory.
Sarkar, A et al., Psychobiotics and the Manipulation of Bacteria–Gut–Brain Signals. Trends in Neurosciences November 2016; 39, (11), 763–781
Evrensel, A, and Ceylan, ME. The Gut-Brain Axis: The Missing Link in Depression. Clinical Psychopharmacology and Neuroscience 2015; 13(3): 239-244.
Magnusson, KR et al., Relationships between diet-related changes in the gut microbiome and cognitive flexibility. Neuroscience 2015 Aug 6; 300:128-40.