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Mental Health, Free Will, and Your Microbiome

“We are legion, each and every one of us. Always a ‘we’ and never a ‘me.’”

Mental Health, Free Will, and Your Microbiome

“I have observed many tiny animals with great admiration,” Galileo marveled as he peered through his microscope — a tool that, like the telescope, he didn’t invent himself but he used with in such a visionary way as to render it revolutionary. The revelatory discoveries he made in the universe within the cell are increasingly proving to be as significant as his telescopic discoveries in the universe without — a significance humanity has been even slower and more reluctant to accept than his radical revision of the cosmos.

That multilayered significance is what English science writer and microbiology elucidator Ed Yong explores in I Contain Multitudes: The Microbes Within Us and a Grander View of Life (public library) — a book so fascinating and elegantly written as to be worthy of its Whitman reference, in which Yong peels the veneer of the visible to reveal the astonishing complexity of life thriving beneath and within the crude confines of our perception.

Early-twentieth-century drawing of Radiolaria, one of the first microorganisms, by Ernst Haeckel
Early-twentieth-century drawing of Radiolarians, some of the first microorganisms, by Ernst Haeckel

Artist Agnes Margin memorably observed that “the best things in life happen to you when you’re alone,” but Yong offers a biopoetic counterpoint in the fact that we are never truly alone. He writes:

Even when we are alone, we are never alone. We exist in symbiosis — a wonderful term that refers to different organisms living together. Some animals are colonised by microbes while they are still unfertilised eggs; others pick up their first partners at the moment of birth. We then proceed through our lives in their presence. When we eat, so do they. When we travel, they come along. When we die, they consume us. Every one of us is a zoo in our own right — a colony enclosed within a single body. A multi-species collective. An entire world.


All zoology is really ecology. We cannot fully understand the lives of animals without understanding our microbes and our symbioses with them. And we cannot fully appreciate our own microbiome without appreciating how those of our fellow species enrich and influence their lives. We need to zoom out to the entire animal kingdom, while zooming in to see the hidden ecosystems that exist in every creature. When we look at beetles and elephants, sea urchins and earthworms, parents and friends, we see individuals, working their way through life as a bunch of cells in a single body, driven by a single brain, and operating with a single genome. This is a pleasant fiction. In fact, we are legion, each and every one of us. Always a “we” and never a “me.”

There are ample reasons to admire and appreciate microbes, well beyond the already impressive facts that they ruled “our” Earth for the vast majority of its 4.54-billion-year history and that we ourselves evolved from them. By pioneering photosynthesis, they became the first organisms capable of making their own food. They dictate the planet’s carbon, nitrogen, sulphur, and phosphorus cycles. They can survive anywhere and populate just about corner of the Earth, from the hydrothermal vents at the bottom of the ocean to the loftiest clouds. They are so diverse that the microbes on your left hand are different from those on your right.

Illustration by Emily Sutton from Tiny Creatures: The World of Microbes by Nicola Davies

But perhaps most impressively — for we are, after all, the solipsistic species — they influence innumerable aspects of our biological and even psychological lives. Young offers a cross-section of this microbial dominion:

The microbiome is infinitely more versatile than any of our familiar body parts. Your cells carry between 20,000 and 25,000 genes, but it is estimated that the microbes inside you wield around 500 times more. This genetic wealth, combined with their rapid evolution, makes them virtuosos of biochemistry, able to adapt to any possible challenge. They help to digest our food, releasing otherwise inaccessible nutrients. They produce vitamins and minerals that are missing from our diet. They break down toxins and hazardous chemicals. They protect us from disease by crowding out more dangerous microbes or killing them directly with antimicrobial chemicals. They produce substances that affect the way we smell. They are such an inevitable presence that we have outsourced surprising aspects of our lives to them. They guide the construction of our bodies, releasing molecules and signals that steer the growth of our organs. They educate our immune system, teaching it to tell friend from foe. They affect the development of the nervous system, and perhaps even influence our behaviour. They contribute to our lives in profound and wide-ranging ways; no corner of our biology is untouched. If we ignore them, we are looking at our lives through a keyhole.

Illustration by Alice and Margin Provensen from The Provensen Book of Fairy Tales

Kafka believed that we look at life through the narrow keyhole of our personal existence and in order to distinguish between appearance and reality, we “must keep the keyhole clean.” Yong performs a masterful act of keyhole-cleaning in demonstrating just how intimately entwined our personal existence is with that of the microbes that inhabit our bodies — a relationship nowhere more counterintuitive yet rife with promise than when it comes to our mental health. It’s hardly instinctive to consider that biology, much less microbiology, can influence the seething cauldron of mental and emotional experience we call psychology. And yet given the centrality of microbes to our immune system microbes and the constant dialogue between our immune system and our central nervous system in shaping our susceptibility to stress and burnout, it pays to probe how our microbiome might interact with our mental health.

Yong notes that research into this question is still in its nascency, so most studies are small and inconclusive, but he points to several curious and promising strands of research. One fMRI study by Kirsten Tillisch found that women who consumed a microbe-rich yoghurt displayed less activity in brain areas implicated in processing emotions, compared to those who consumed a microbe-free yogurt. In a clinical trial by Stephen Collins for patients with irritable bowel syndrome, a probiotic bacterium reduced symptoms of depression. Psychiatrist Ted Dinan, who runs a clinic for patients with depression, is wrapping up a clinical trial on “psychobiotics” — probiotics that might help people manage stress and depression. Although Dinan himself is skeptical that such treatments would be effective for those with debilitating clinical depression, he is hopeful that people with milder mood disorders might find some relief.

Art by Bobby Baker from Diary Drawings: Mental Illness and Me

But the most striking implication of even the very possibility that microbes might shape our moods is that they might also shape our choices and, in consequence, our very destinies. Yong considers the overwhelming range of imputations:

These studies are already forcing scientists to view different aspects of human behaviour through a microbial lens. Drinking lots of alcohol makes the gut leakier, allowing microbes to more readily influence the brain — could that help to explain why alcoholics often experience depression or anxiety? Our diet reshapes the microbes in our gut — could those changes ripple out to affect our minds? The gut microbiome becomes less stable in old age — could that contribute to the rise of brain diseases in the elderly? And could our microbes manipulate our food cravings in the first place? If you reach for a burger or a chocolate bar, what exactly is pushing that hand forward? From your perspective, choosing the right item on a menu is the difference between a good meal and a bad one. But for your gut bacteria, the choice is more important. Different microbes fare better on certain diets. Some are peerless at digesting plant fibres. Others thrive on fats. When you choose your meals, you are also choosing which bacteria get fed, and which get an advantage over their peers. But they don’t have to sit there and graciously await your decision. As we have seen, bacteria have ways of hacking into the nervous system. If they released dopamine, a chemical involved in feelings of pleasure and reward, when you ate the ‘right’ things, could they potentially train you to choose certain foods over others? Do they get a say in your menu picks?

These questions flirt with the conundrum of free will by making us contend with the discomfiting notion that each of us might after all be what neuroscientist Sam Harris has called “a biochemical puppet.” And although these puzzlements are still largely in the realm of the hypothetical, Yong points out that such dependencies are far from uncommon in nature. He writes:

Nature is full of parasites that control the minds of their hosts. The rabies virus infects the nervous system and makes its carriers violent and aggressive; if they lash out at their peers, and inflict bites and scratches, they pass the virus on to new hosts. The brain parasite Toxoplasma gondii is another puppetmaster. It can only sexually reproduce in a cat; if it gets into a rat, it suppresses the rodent’s natural fear of cat odours and replaces it with something more like sexual attraction. The rodent scurries towards nearby cats, with fatal results, and T. gondii gets to complete its life cycle.

The rabies virus and T. gondii are outright parasites, selfishly reproducing at the expense of their hosts, with detrimental and often fatal results. Our gut microbes are different. They are natural parts of our lives. They help to construct our bodies — our gut, our immune system, our nervous system. They benefit us. But we shouldn’t let that lure us into a false sense of security. Symbiotic microbes are still their own entities, with their own interests to further and their own evolutionary battles to wage. They can be our partners, but they are not our friends. Even in the most harmonious of symbioses, there is always room for conflict, selfishness, and betrayal.

In the remainder of the intensely interesting I Contain Multitudes, Yong goes on to explore how these lines are drawn and what we can do to make the most of those alliances. Complement it with Tiny Creatures — a lovely children’s book primer on the universe of microbes — then grow agape at Yong’s terrific and slightly terrifying TED talk about mind-controlling parasites:

Published August 10, 2016




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