Bacteria & Serotonin

What is the connection between serotonin and gut bacteria?
Isn’t serotonin just that “happy chemical” found in our brains?
 
Actually, the majority of our serotonin is found in our intestines! There are some little cells in the intestinal tract called “enterochromaffin cells” which is where we find most of it. But in order for these cells to produce serotonin, they need some help from the friendly bacteria that also live in our intestines. 
Did you know an adult human can have more than a kilogram of bacteria in their gut?! It’s a good thing too! Researchers have experimented with mice that don’t have any gut bacteria and have noticed much lower serotonin levels than mice who do. They also found out that if they re-establish their microflora, their serotonin levels return!

But what is serotonin good for anyway? 
In the gut, it stimulates motility or movement, it promotes the growth and maintenance of the hugely important gut mucosa (which interestingly replaces itself every 48hours), it promotes neurogenesis, modulates the gut inflammatory response, influences osteoblastic activity, promotes hepatic regeneration, and impacts the development and survival of dopaminergic neurons in the enteric (gut) nervous system (to name a few things!). Outside of the gut, serotonin has a couple more minor roles ;), such as the regulation of our mood and of our sleep -wake cycle. 
Are you wondering by now how the serotonin in our intestines can affect our brain? It starts with activation from our autonomic nervous system (the same nervous system that tells our lungs to breath in and out). This causes the serotonin to be released from the cells in our gut. Then the enteroendocrine gut cells release some signals that can finally be used to communicate with the central nervous system (brain) via the large vagus nerve. Amazing stuff right?
 
So what do you think? Are gut bacteria important? Do we want a kilogram of microbiota living in us? I’d say so! And this is only one of the reasons why a strong microbiome is important.
However, sometimes our gut microbiota can become depleted or out of balance due to things like diet, antibiotic use, drug exposure, or stress......