Great Throughts Treasury

"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 puppet-master. It can only sexually reproduce in a cat; if it gets into a rat, it suppresses the rodent?s natural fear of cat odors 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."

"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 colonized by microbes while they are still unfertilized 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.?"

"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."

"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."

"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?"