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The Microbes that Make Us Human

If you ever gotten glitter on you, then you know that you will inevitably continue to find glitter on you many days after you’ve washed and showered.

Take a moment to think about what that means about the germs that get on your skin…

Gross, right?

Well, not entirely.

The microbes that live on your skin, in your intestines, and in your respiratory tract are important for your health! New research suggests that the microscopic organisms that make up your gut microbiota can even affect your mood and behavior! In essence, the very things that make you you are in part thanks to the microorganisms living inside you.

What is the Gut Microbiota?

The gut microbiota is the name given to the hundreds of trillions of microorganisms that live in the gastrointestinal tract. The two primary microbe populations that account for roughly 90% of the gut microbiota are of the genus Bacteroides and the phylum Firmicutes.

Normally, these microbes coexist peacefully inside their host, but only if they are at a specific ratio. Sometimes, an imbalance of the microbiota population occurs, and this dysbiosis can cause a variety of illnesses due to a lack of diversity or a shift toward an increase in pathobionts (disease-causing microbes). Dysbiosis has been shown to play a part in many intestinal disorders, such as inflammatory bowel disorder, Crohn’s Disease, and colon cancer, as well as metabolic health problems, such as obesity, metabolic syndrome, and diabetes, in addition to psychiatric disorders, such as depression, anxiety, and even late-onset forms of autism spectrum disorders! Even in your everyday life, the gut microbiota plays a role in affecting you mood and behavior. Put simply, the things that seems so inherently part of human nature are actually influenced by the small organisms that live inside our bodies.

The Gut-Brain Interaction

Our behavior is regulated by our central nervous system which is made up of our brain and spinal cord. This raises the questions: How can something all the way down in the gut can have an effect on the central nervous system? How do these two systems communicate?

The gut microbiota is able to affect our behaviors and mood through interactions with our central nervous system in a relationship called the gut-brain axis. This relationship flows in both directions: the central nervous system regulates neurochemicals that allow the two body systems to interact, and the gastrointestinal tract can produce a variety of molecules that can affect neuronal circuits, which ultimately change our behaviors and moods.

Lessons from Mice

Though the number of human studies are limited, we can learn a lot from studies on our the model organism, the mouse, which experiences similar diseases and exhibits behaviors that relate to human behaviors. Studies in mice show that stress, anxiety, and depression levels can change by adding certain beneficial probiotics to their diet. What’s more, even changes in diet alone can cause changes in behavior by altering the composition of the microbiota.

Several mouse studies have investigated the relationship between gut microbiota and anxiety-like behavior in mice. One study looked at germ-free mice, since these mice are essentially free of all microbes. The researchers wanted to know how animals with no microbiota behaved compared to regular animals. The germ-free mice were tested using a variety of anxiety tests and tests for locomotor activity using elevated-plus maze and light-dark box test (see the below figure adapted and modified from Remarkably, germ-free mice were less anxious and more motile compared to mice with normal gut microbiota, suggesting that microbial populations can affect neural signals involved in anxiety behavior and motor control.

Another study used a different strain of mice that naturally exhibits anxiety-like behavior. After giving the mice oral antimicrobials to change the composition of the microbiota, the researchers analyzed the anxiety levels in these mice. Amazingly, these mice were less anxious and more curious, showing an increase in exploratory behavior. These results suggest that changing the microbiota composition is an important factor in producing behavioral changes.

So what happened when germ-free anxious mice were given microbes from a normal, anxiety-free mouse? The germ-free anxious mice became less anxious! Conversely, when the researchers colonized the normal germ-free mice with the microbiota of the anxious mice, the mice became more anxious, and there was a notable reduction in their exploratory behavior. In other words, the behavior of the mice changed to match their microbiota.

The results of these mouse studies highlight just how much the gut microbiota can influence mood and behavior. But is the same true for humans?

Mental Therapy through Microbes

While creating a germ-free human participant is impossible, researchers have studied the effects of giving a probiotic to patients who suffer from chronic fatigue syndrome. People who suffer from this illness frequently report high levels of anxiety. After eight weeks of taking the probiotic, patients reported a significant decrease in anxiety symptoms. Clearly, probiotics can have profound positive effects on people’s moods.

“Put simply, the things that seems so inherently part of human nature are actually influenced by the small organisms that live inside our bodies.”

In a similar study done on healthy participants from the general population, people who took two probiotics for thirty days also experienced a decrease in depression and anger/hostility compared to the beginning of the trial. In addition, people who had felt moderately stressed at the beginning of the trial also reported improved moods after probiotics. In another study, healthy participants consumed a milk drink containing one probiotic for three weeks. Those who were considered more depressed before beginning the trial reported increased levels of happiness after taking the probiotic. Together, these results suggest that probiotics not only reduce anxiety, but they also promote happiness.

These studies pose an interesting rebuttal to the overwhelming assumption that germs are bad. Perhaps we are using, hand sanitizer and broad-spectrum antibiotics when we don’t necessarily need to. If taking a probiotic for a few weeks can alter the gut microbiota enough for us to see changes in our mood and behavior, then our microbiota isn’t simply a passenger in our body, sitting around in our gut. Rather, our gut microbiota is actually responding to and interacting with the medications we take or the foods we eat.

Are Brain Foods Actually Microbiota Foods?

If the right microbiota can make us happier, can others make us smarter? A study in mice suggest that they do affect cognitive abilities like learning and memory. In this study, mice received a diet containing 50% lean ground beef as opposed to a regular rodent chow for three months and then were tested on their learning and memory abilities. The researchers found that the mice on the lean ground beef diet had an altered gut microbiota population and a better working and reference memory than the mice fed the regular chow. To top it all off, these mice also displayed reduced anxiety during testing.

Another group of researchers studied the effects of dietary soy phytoestrogens on behavior. A lot of people, who are on vegetarian or vegan diets or consume dietary supplements, consume soy-based foods, which carry phytoestrogens. Evidence suggests that phytoestrogens may lower rates of osteoporosis, heart disease, breast cancer, and menopausal symptoms, but they may also cause health problems. In order to understand the importance of phytoestrogens in shaping healthy diets, researchers studied mice on a phytoestrogen diet. They found that these mice had less anxiety. Moreover, researchers found that female mice that consumed phytoestrogens had improved visual-spatial memory, but male mice that consumed phytoestrogens actually had diminished visual-spatial memory abilities, suggesting a possible sex factor in the gut-brain relationship. At this time, no study has been done in humans to see if this diet affects our cognitive abilities. However, it seems plausible that if so-called “brain foods,” such as avocados, fish, berries, and red wine, actually boost focus and memory, then perhaps they do this by promoting the health of our microbiota and supporting certain microbes that improve our cognition through the gut-brain axis.

When you are thinking about your own health, remember the microbes that live with you. They make up a large part of who you are and how you act.


Written by Shuhan He in collaboration with Maze Engineers

Images by Maze Engineers.



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  • Shuhan He

    Shuhan He is the founder of MazeEngineers. He is a resident physician at the Harvard Emergency Medicine at Brigham and Women's and Massachusetts Hospital, and graduated with his MD from the Keck School of Medicine. He currently works with researchers across the world to develop better objective preclinical testing. His dream is that good, mass behavior investigations can help bring new therapies to the bedside. He can be found on twitter at @ShuhanHeMD.

Shuhan He

Shuhan He is the founder of MazeEngineers. He is a resident physician at the Harvard Emergency Medicine at Brigham and Women's and Massachusetts Hospital, and graduated with his MD from the Keck School of Medicine. He currently works with researchers across the world to develop better objective preclinical testing. His dream is that good, mass behavior investigations can help bring new therapies to the bedside. He can be found on twitter at @ShuhanHeMD.

2 thoughts on “The Microbes that Make Us Human

  • Ken Seidel

    Perhaps it’s best we think of ourselves as a poly-parasitic species, they living off us and us living off them? Or, perhaps we are simply the mutated results of cohabiting species? And, perhaps our ego bound self reflections cause us to dismiss our interdependence so we attempt to extinguish all other creatures as if they were competitors showing them who is boss. What do u think?

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