The 'Gut-Brain Axis' and the Benefits of Bacteria

Our individual 'microbiomes' may be key to good health

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Riccardoariotti/WIKIMEDIA COMMONS

SiS is proud to feature the winners of the "2014 Driskill Graduate Program in the Life Sciences (DGP) Science and Society Class Distinction Award." Written as part of a course on science and society, these papers were chosen by DGP faculty and SiS staff to be published in HELIX. This month, we present the following piece by MD/PhD student Daniel W. Fisher.

When describing what makes someone who they are, we probably start by talking about their physical attributes: their steely blue eyes, raven-black hair, or one lonely heart-shaped freckle below the bottom eyelid of their left cheek. But, more often than not, we’d stop and say that our looks really aren’t what make us unique. After all, a few nips and tucks can change a lot of what we see in the mirror.

Instead, we’d say it’s our personalities that define us. We’d tell you about someone being "a wild one" or, perhaps, "quiet and bookish". We’d tell you about her love of tulips or his desire to be a famous jazz pianist someday. Eventually, the savviest among us would realize that we are both the physical and mental descriptions of ourselves, claiming it’s only when we realize both of them that we begin to understand who we are.

However, while it’s true that we are our bodies and brains, we may also be more bacteria than we ever thought.

The human body is host to approximately more than 10 times as many bacterial cells as human. And, while the folks at Lysol and Purell may want us to believe that all bacteria are out to get us, the truth is that the vast majority are either completely harmless or even beneficial. Without these little single-celled organisms, we‘d be unable to digest, extract and synthesize nutrients from food. What’s more, bacteria can help us stay healthy, as “good” bacterial strains outcompete others that cause disease. Without these bacteria, our immune systems would be overwhelmed.

Scientists refer to all the bacteria that live inside an individual as their “microbiome.” Much like a rainforest is made up of thousands of plants and animals, the microbiome is made up of hundreds of different bacterial species. Similarly, as a rainforest in South America is vastly different than a rainforest in Africa, so is the microbiome of one person different from the next.

Researchers are now finding that these differences in microbiomes are important to understanding health and disease. Consequently, a new field of bacterial research has emerged that may describe the most exciting functions of these essential symbiots yet. It turns out that the bacteria in our gut may have a significant and direct influence on how our brains work, through the newly discovered “Gut-Brain Axis.”

The Gut-Brain Axis describes a path from the intestinal tract to the central nervous system through the"vagus nerve (also known as the “wandering” nerve because of its long, circuitous track through the body). While for many years it’s been known that the Vagus Nerve can cause changes in digestion, only recently has evidence suggested that the gut could directly send signals to the brain.

For example, research done by a Canadian team at McMaster University in Ontario has shown that some gut bacteria create specific neurotransmitters – or chemical brain signals – that were found to alleviate anxiety in mice. To demonstrate this, the researchers gave the mice a cocktail of antibiotics to prune certain organisms from their microbiome (again, fancy word meaning ‘bacteria inside them’) and tested their behavior. They found that these mice were more likely to go into the lit area of a small maze than their non-treated counterparts. Instinctually, more fearful mice will avoid light, so this test is often used as an indicator of a mouse’s anxiety-like behavior.

While interesting, it is possible that the antibiotics themselves actually altered the brain. So, to prove it was caused by changing the bacterial species, they transplanted the intestinal bacteria from “brave” mice into the digestive tracts of more timid ones. Amazingly, they found that these more timid mice would now explore the lit areas, too!

To describe how this might work, these researchers showed that there was more of a special neurotransmitter called "Brain-Derived Neurotropic Factor (BDNF)" in the brains of the treated mice. In concert, it was also found that the bacteria made BDNF in those mice’s guts. What makes this increase in BDNF so exciting is that patients with anxiety and depression have decreased BDNF. Further, many antidepressants have been found to increase BDNF. What’s more, other neuropsychiatric diseases like schizophrenia and Alzheimer’s Disease also have decreased BDNF levels similar to depression.

These results have since been supported by others. One group showed that mice born in a completely sterile environment – and so uninhabited by any bacteria – had more stress hormones when challenged with a stressor. When strains of “good” bacteria were given to these mice, they had a decreased release of these hormones. Another group showed that mice given the bacteria Lactobacillus rhamnosus were less depressed and anxious than their counterparts, but only when the vagus nerve was intact. These changes were mediated by differences in the expression of select genes known to be involved in psychiatric disorders.

But, these data aren’t really that surprising. It has already been shown that specific microbiome compositions correlate well with certain diseases such as diabetes, obesity and even eczema. So, it’s not much of a leap to posit that bacteria can also influence mood. But, it’s important to be cautious before accepting these results as truth. For one thing, just because there are changes in mice, it doesn’t mean they extend to humans.

Still, it’s OK to imagine the possibilities. Even within the medical community, the topic of a “fecal transplant” is starting to be discussed more and more. For this procedure, you would move the loose matter in the gut from one person to another, thus transplanting the microbiome of a healthy individual to one who is ill, with the goal of curing them.

Admittedly, this operation would need a major PR revamp. The words “fecal transplant” or “the-delivery-of-healthy-poop” may be slightly off-putting for many patients. Still, the applications of this procedure could be incredibly transformative for modern medicine. What if physicians could slow the progression of neurodegeneration by giving a patient “pro-brain” poop (or bacteria)? Indeed, there have been some anecdotes of more unconventional physicians using fecal transplants to treat multiple sclerosis, a debilitating movement disorder that manifests from the immune system and attacks the insulation around the nerves.

Perhaps, in the future, psychiatrists might even be able to change our moods or improve our thinking by simply changing our microbiomes. To the dreamer, the possibilities are as vast as the many bacteria that co-habit our bodies. Still, while exciting, more focused research will be needed to separate what is fact from fiction. But, in time, we may just start to describe our selves not only by what’s in our heads but also by what’s in our guts. 

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