Who Are You? A Glimpse Into The Human Microbiome
Who are you? It is a seemingly simple question; yet, the topic of self concept is one that has been pondered for centuries. If your legs are swapped for someone else's, are you still yourself? What about your arms or kidneys or liver or heart? Have you considered who you would be if your brain was swapped for someone else’s brain?
While questions of such a type are indeed complicated, to say the least, I’d like to focus on a bodily swap of a different category: the microbes that reside within you. For, although you may snicker, current data estimates the number of microbes within the human body to be roughly 40 trillion, as compared with our measly 30 trillion human cells. Granted, this approximation is down from the recently considered ratio of 10:1, microbe cells to human cells. Nevertheless, while the precise number is yet to be worked out, this idea carries an interesting notion: by biological unit, you are likely more bacteria than you are human.
Until recently, the unique array of microbes living inside us were considered simply as passengers along for the ride of human life. However, scientific research has begun to uncover an intriguing aspect to our partners in crime -- that is, the bacteria residing within and on our bodies have the ability to alter our biology and physiological systems.
The microbes’ connection to our brain, for example, is of the most heavily studied. The connection provides insight into the ways in which bacteria can affect our psychology. The interaction between our gastrointestinal system and the brain is termed the gut-brain axis and it is far from a one-way street. The brain talks to the bacteria in the gut, and the bacteria talk back to the brain. This is all, of course, in the form of chemical messengers. But without getting into the precise mechanism of how a bacterium might use protons, neutrons, and electrons to speak, I’d like to focus on the macroscopic conclusions of this idea.
The vagus nerve (which travels down from the brainstem into our bodies) can be considered as a communication highway of sorts between our brain and many organs of the body. Researchers have uncovered the ability of microbes in our guts to stimulate the vagus nerve, thereby providing a communication pathway to the brain (Forsyth et al, 2014). So too, microbes have the ability to activate this vagal pathway thereby providing a mechanism for their communication with the brain.
Multiple studies have been performed in an attempt to see how the microbes living inside us can theoretically alter our psychology. For example, a 2014 study published in the journal Psychoneuroendocrinology found that germ free mice (mice born and raised without any bacteria inside of them) responded more strongly to acute stressors and general anxiety stimuli (Crumeyrolle-Arias, 2014).
Studies on depression have seen similar results (yes, there are ways to tell if a mouse is depressed!). In one of the most fascinating studies on this subject (which simultaneously demonstrates both the complicated nature of human physiology as well as our current ignorance on this particular subject), researchers extracted the microbiota of 34 humans with major depressive disorder and transplanted their microbiomes into germ free rats. In comparison with the healthy control group, the rats with the “depression microbiomes” exhibited anhedonic (lost of interest) behavior. How is that possible? In short, while the details are still being worked out, certain strains of bacteria are thought to be communicating with our brain through molecules quite familiar to us -- neurotransmitters. As it would seem, our distinctly harbored microbiota influence our brains, and therefore our psychology.
But it doesn't just stop at depression and anxiety. Microbiota transplants are now being referred to by some as “personality transplants”. The neurotransmitters that determine our mood state, as incredibly complex as the system may be, are seemingly influenced by our personalized microbiomes. Easy-going mice have different microbiomes than nervous mice. High cognitively scoring human babies have different microbes than low scoring babies (Alexander L. Carlson). This notion has been replicated again and again. At this point, however, our collective knowledge is premature on this subject. So too, the direction of causality has yet to be worked out. Do microbes contribute to depression, or does depression change our microbiomes? Time will undoubtedly uncover the precise mechanisms of this dialogue between brain and bacteria. What we do know, however, is that microbiome diversity is a common denominator in most advantageous psychological characteristics.
How does one then acquire his or her distinct and diverse set of microbes? There are many different factors, including whether or not you were delivered by cesarean section, how often you take antibiotics, and perhaps even your dog at home, with which you may be sharing microbes. However, one of the easiest ways to acquire a diverse set of microbes is to provide the bacteria a diverse set of food sources. What exactly do your gut bacteria eat? Fiber, and lots of it. According to an immense body of literature, including a 2017 study published in the Journal of Obesity, fiber intake was positively correlated with human microbiome diversity (C, Menni). Moreover, a 2016 study published in the journal Gut, showed that individuals following a vegan diet (a diet naturally high in fiber), demonstrated the most diverse microbiota profiles as compared with their omnivorous counterparts (Francesca De Filippis, et al). Multiple other studies have replicated these results.
Evidently, microbiologically altered neurophysiology is quite complicated. So too, the field lends particular interest to the area of philosophy.
I therefore ask again: who are you? Are you your brain? If you are your brain, and your brain is influenced by your microbiome, are you your bacteria?
If I swapped out your bacteria for someone else's, are you still yourself?