Microbes help to shape and maintain our bodies

Reader, as you read these words, trillions of microbes and quadrillions of viruses are multiplying on your face, your hands and down there in the darkness of your gut. With every breath you take, with every move you make, you are sending bacteria into the air at the rate of about 37 million per hour — your invisible aura, your personal microbial cloud. With every gram of food you eat, you swallow about a million microbes more.

According to the latest estimates, about half of your cells are not human — enough to make you wonder what you mean by “you”. Your human cells come from a single fertilised egg with DNA from your mother and father. Microbes began mingling with those human cells even before your first breath, the first kiss from your mother, your first taste of milk. And your human cells could not have built a healthy body without intimate help from all those trillions of immigrant microbes — your other half.

“I am large, I contain multitudes,” Walt Whitman declares in “Leaves of Grass”, in his great poem “Song of Myself”. But what is “self”? According to conventional wisdom, your immune system is supposed to protect you by detecting and rejecting anything in your body that is not “self”. And yet your very immune system is partly built and even partly run by microbes. “Even when we are alone, we are never alone,” Ed Yong writes in his excellent and vivid introduction to our microbiota, or microbiome, the all-enveloping realm of our microbes. “When we eat, so do they. When we travel, they come along. When we die, they consume us.”

“I Contain Multitudes” has a terrific story to tell. For the last quarter-century or so, microbiologists have been exploring what may amount to a new view of life, full of fascination and self-contradiction. Their work suggests strange and surprising things about health and disease, about symbiosis and risk. This is one of the most interesting developments in biology today. It sweeps from the personal to the planetary; it changes the way you look at human bodies, birds in the air and leaves of grass. Like all new views, it is hard to take in — although, like it or not, we can hardly get away from it.

In a way, Yong says, the science of the microbiome had two false starts. First the Dutch draper and microscope maker Anton van Leeuwenhoek discovered that microbes, which he called animalcules, are everywhere — on skin, on wood, on fur, on eyes. In 1683, he scraped a bit of plaque from his own teeth and inspected it through the microscope. He saw enormous numbers of living things “very prettily a-moving”. Van Leeuwenhoek estimated, correctly, that there were more animalcules swimming and creeping around in his mouth than there were people living in the Netherlands. Unfortunately, many of his sensational discoveries turned out to be difficult to replicate because no one else could see the animalcules as clearly as he did. Van Leeuwenhoek was such a wizard with a microscope that he was, in a sense, looking far into the future.

Two hundred years later, in the second half of the 19th century, microbial life caused a second sensation. Louis Pasteur, Robert Koch, Joseph Lister and others focused the world’s attention on microbes’ ability to cause diseases. The “germ theory” became the main lens through which the world viewed microbes. At that time, as Yong reminds us, naturalists were trying to take in Darwin’s view of life. The germ theory fit in well with the struggle for existence, the survival of the fittest. Nature was red in tooth and claw, and danger lurked between our teeth.

And of course that’s the way most of us still think about bacteria now. Our whole worldview is antibacterial. They’re out there. Shoot to kill. Build a wall! Which makes perfect sense for the tiny percentage of microbes that make us sick, but misses the vast majority that make us what we are. “If we ignore them,” Yong writes, “we are looking at our lives through a keyhole.”

The view got wider in the late 20th century, when pioneering microbiologists began exploring the microcosmos using the new technologies of gene sequencing and genomics. With these tools, they discovered that they could sequence every single gene in a sample from a streambed, a skin flake, a cheek swab. That gave them an instant, Olympian view of myriad microbes at once. Most of them were unknown to science. Suddenly researchers were adding new branches and twigs to the tree of life almost everywhere they looked. “From the very first days of this,” one of the pioneers, Norman Pace, tells Yong, “we just blew open the doors of the natural microbial world. I want that on my epitaph. It was a wonderful feeling and still is.”

In 1999, for instance, the microbiologist David Relman found a surprising number of new strains of bacteria in plaque from between his own teeth, even though the human mouth was, by then, the best-studied place in microbiology. In 2005, Relman found almost 400 species of bacteria in the human gut, and most of those had never been seen before either. Menageries living at the start of the small intestine are different from the micropopulations that live down at the anus. Different creatures live above and below the gumline. Only one-sixth of the bugs on your right palm are the same species as the bugs on your left. Oenophiles know that grapes in a vineyard on one slope can be different from grapes on the facing slope; in the same way, for microbes, the terroir of your left armpit is different from the terroir of your right, with different species and varieties growing in each.

Human cells and microbial cells are incredibly interdependent, because we have evolved together. We provide their habitats; they provide their labour. Actually, Yong writes, “they are more management than labour”. The human genome consists of about 25,000 genes. But the combined genomes of all of our fellow travellers (some microbiologists call them our “old friends”) are about 500 times larger. At the biochemical level they are much more nimble and versatile than we are. Our bodies, Yong writes, “are continuously built and reshaped by the bacteria inside us”. This kind of profound symbiosis has probably been going on since the appearance of the first multicellular animals. “Perhaps,” Yong writes, “it is less that I contain multitudes and more that I am multitudes.”

Ed Yong is a talented British science writer, a staff writer for the “Atlantic” and the author of a wonderful blog, Not Exactly Rocket Science, hosted by National Geographic. “I Contain Multitudes”, his first book, covers a huge amount of microscopic territory in clear, strong, often epigrammatic prose. Yong has advanced degrees in biology, and he is remarkably well informed; he includes descriptions of many studies that are still unpublished, and even a few original ideas for new experiments. He is infectiously enthusiastic about microbes, and he describes them with verve. A larval Hydroides elegans looks like “a wall plug with eyes”. A choanoflagellate, which is the closest living relative of every creature in the animal kingdom, looks “like a sperm wearing a skirt”. Even the book’s endnotes are rich with interesting asides, swarming with interesting sidelights, a teeming microbial world.

This is the world you live in. This is the skin you live in. Make yourself at home.

–New York Times News Service

Jonathan Weiner’s books include “Long for This World” and “The Beak of the Finch”, winner of the Pulitzer Prize.