Researchers examined the relationship between gut microbiomes and depression. To do so, they collected fecal samples from people with depression and also a control group of individuals with no signs of depression. They then performed a fecal microbiota transfer (FMT) by adding the samples to rats that had no gut microbiota and examined behaviors associated with depression and anxiety as well as species of bacteria that ended up growing in the rats’ guts. The results are presented below. Graph (a) shows the rats’ interest in a pleasurable experience (drinking sugar water). Graph (b) shows the amount of time rats spent out in the open versus along the edge of an area (a sign of anxiety). Graph (c) shows the number of species observed in rats after FMT.





Hypothesize how bacteria living in a person’s gut might influence events occurring in the brain.

Verified step by step guidance

1. Examine Graph (a) which shows the sucrose preference of rats. Note that rats with fecal samples from depressed individuals show a lower preference for sucrose compared to controls.
2. Analyze Graph (b) which depicts the open field test results. Observe that rats with fecal samples from depressed individuals spend less time in the center, indicating higher anxiety levels.
3. Look at Graph (c) which illustrates species richness. Notice that rats with fecal samples from depressed individuals have a lower number of observed species compared to controls.
4. Hypothesize that the gut microbiota from depressed individuals may influence brain function by altering the gut-brain axis, potentially affecting neurotransmitter levels or inflammation.
5. Consider that changes in gut microbiota composition could lead to behavioral changes in the rats, such as decreased pleasure-seeking behavior and increased anxiety, mirroring symptoms of depression in humans.

Key Concepts

Gut Microbiome

The gut microbiome refers to the diverse community of microorganisms residing in the gastrointestinal tract. These microbes play a crucial role in digestion, metabolism, and immune function, and emerging research suggests they may also influence mental health by producing neurotransmitters and other metabolites that can affect brain function and behavior.

Fecal Microbiota Transfer (FMT)

Fecal microbiota transfer (FMT) is a medical procedure that involves transferring fecal matter from a healthy donor to a recipient to restore the recipient's gut microbiome. This technique is being studied for its potential to treat various conditions, including gastrointestinal disorders and mental health issues, by reintroducing beneficial bacteria that may be lacking in the recipient's gut.

Behavioral Indicators of Depression and Anxiety

Behavioral indicators of depression and anxiety in animal models, such as rats, can be assessed through tests like the sucrose preference test and the open field test. The sucrose preference test measures an animal's interest in pleasurable stimuli, while the open field test evaluates anxiety levels based on the time spent in open versus enclosed spaces, providing insights into the emotional state of the subjects.