More people have come to know that a healthy balance of trillions of microscopic organisms in the human gut can be useful for good health. A new study by scientists at the University of California, Los Angeles (UCLA) suggests that serotonin, and drugs that affect its levels, can have impact on this balance.
The scientists discovered a less-known bacterium called Turicibacter sanguinis, capable of detecting and transporting serotonin into bacterial cells. They found that the organism’s ability to move the chemical messenger into cells reduced in the presence of the antidepressant fluoxetine, popularly known under the brand name Prozac.
Serotonin is a neurotransmitter that helps to send messages between and among cells. Many people know it for its effects on mood and emotions. It is one of the major so-called “feel good” chemicals.
The bulk of serotonin produced in the body (up to about 90 percent) comes from the gut. It is highly crucial there for immunity.
Findings from this new research were published in Nature Microbiology. The study was led by Dr. Elaine Hsiao, UCLA assistant professor of integrative biology and physiology, and Thomas Fung, a postdoctoral fellow.
Serotonin increases number of bacteria
Hsiao’s team had reported in a 2015 study which appeared in the journal Cell that a bacterial mixture, comprising mainly Turicibacter sanguinis and Clostridia, led to increased production of serotonin in mice.
The levels of the chemical, on the other hand, fell by more than half in the gut of the animals when the bacteria were missing. These rose to normal levels after the same mixture was added in the mice.
In the latest study, the UCLA scientists sought to find out why the bacteria might cause higher serotonin production. They wanted to know why the microorganisms produce molecules that signal gut cells to make more of the chemical.
To investigate this, the researchers began by adding serotonin to the drinking water of some mice. They genetically engineered some others to increase their gut serotonin levels.
It was observed that the number of the bacteria Turicibacter and Clostridia increased considerably as a result of more serotonin in the gut of the mice.
The scientists also added fluoxetine, a selective serotonin reuptake inhibitor (SSRI), to a tube that contained Turicibacter. That antidepressant is known to impede mammalian serotonin transporter.
It was found that exposure to the drug cause the bacterium to move by far less serotonin into bacterial cells. As a result, the bacterium only grew to low levels in the animals.
“Our new study tells us that certain gut bacteria can respond to serotonin and drugs that influence serotonin, like anti-depressants,” Fung said. “This is a unique form of communication between bacteria and our own cells through molecules traditionally recognized as neurotransmitters.”
How bacteria detect serotonin
Hsiao and her team went further to investigate the mechanism by which the microorganisms were able to sense the neurotransmitter. They did this in collaboration with a co-author of the study Lucy Forrest and her team at the National Institute of Neurological Disorders and Stroke, one of the centers making up the National Institutes of Health (NIH).
It was discovered that several species of Turicibacter has a protein, which is somewhat similar, structurally, to a protein that serves as serotonin transporter in mammals.
Turicibacter sanguinis grown in the lab was found to import serotonin into the cell. But the bacterium was less able to do this after Prozac was brought in.
The findings were in line with previous studies that showed certain bacteria in the gut influence serotonin levels. They also add to growing evidence that antidepressants impact gut microbiota.
Hsiao said what remained was to find out if the effect antidepressants have on bacteria has a link to health and disorders.