Near the turn of the century, researchers discovered that microbes could produce almost every neurotransmitter found in the human brain, including serotonin and dopamine, your feel-good chemicals. The thing is that the brain has this thing called the “blood-brain barrier” to protect it from outside influence. This is supposed to prevent cells, particles and certain molecules—including neurotransmitters—from getting in, but here we had microbes seemingly breaking the rules.

 

It wasn’t until 2017 when researchers finally figured out how the two were connected: by special cells in the gut lining. Said cells can detect neurotransmitters produced by microbes, resulting in a pulse being triggered in the vagus nerves (located in your brain), thus directly connecting the gut to the brain. More evidence suggests that the gut microbes and the molecules they produce can directly modulate the integrity of the blood-brain barrier, too.

 

Researchers have also discovered a link between gut bacteria and the way healthy people process emotions. Kirsten Tillisch and Emeran Mayer, both at the University of California, Los Angeles, examined the gut microbes of 40 women, dividing them into two groups: those with lots of bacteria from the genus Prevotella, and those with lots from the genus Bacteroides.

 

Using MRIs to observe specific parts of the brain while women viewed emotionally disturbing images, researchers discovered that each group had distinct brain activity. The group with plenty of Protella bacteria had less activity in the hippocampus, which is correlated with depression.

 

Tillisch and Mayer took their findings a step further by influencing the way people’s brains processed emotions by feeding them probiotics. Brain scans revealed that ingesting certain probiotics regularly affected the activity and connectivity in the emotion centers in the brain, producing changes associated with healthier emotional processing.