There is a staggering number of cells in a human being. The brain alone is made up of about 100 billion nerve cells. The heart has about two billion muscle cells. We shed rough a half-billion skin cells every day.
And yet, those numbers pale in comparison to the numbers of bacteria, viruses and other microbes that live on — and in — us, every day of our entire lives. What we think of as a “human” body really isn’t very human at all. If anything, we’re mostly an apartment building for the microbes that call us home.
Take the gut…everything from the stomach on down. Research suggests that each of us hosts about 160 species of bacteria in our gut (not counting any fungi or protozoa living in there); the exact list differs from person to person, but more than 1,000 bacterial species are capable of living in the human intestines. Another 1,000 species colonize the skin; the belly button alone harbors 67.
In all, we may each carry upwards of 100 trillion bacterial cells, 10 times the number of human cells in the body. Let me put it another way: that’s over 1,400 times more bacteria in and on you then there are people on Earth.
The majority of the bacteria in and on our body are harmless or even helpful. In the gut, for instance produce proteins and other factors we need, help us digest and absorb nutrients, and close the door on pathogens before they can cause serious harm. Some help train the human immune system, keeping the immune response from going haywire.
Every fold of skin, every twist of intestine, is different microbial neighborhood. And those neighborhoods don’t necessarily look the same from person to person. The Human Microbiome Project, which last year published an extensive (although by no means comprehensive) survey of bacteria from different sites in the skin, mouth, gut, nose, and genitalia of 242 healthy Midwesterners and Texans, found remarkable differences in the bacterial makeup between individual people.
Our bacterial profile also differs over time. When we’re in the womb, we’re bacteria-free, but the first gift we get from mother upon being born is a big dose of E. coli; we pick it up in the birth canal. Additional bacteria get passed on through breast feeding and other environmental exposures until, when we’re around a year or two old, our microbiome settles in. But studies have started to show that our bacterial profile starts to change again as we get old. Changes in diet, disease, and environment can also cause shifts in our bacterial makeup. (And apparently so can competing in a roller derby match.)
But that’s just the bacteria.
The first things that come to many people’s minds when talking about viruses and us is probably something like the word “bad!” But by some accounts, we harbor as much viral diversity in our bodies as we do bacterial.
It’s tough to say exactly what viruses we have within us, though, because it’s much harder to take a census of our viral guests. As science writer Sarah C.P. Williams points out, all bacteria share a gene called 16S rDNA, which helps them make proteins. To look for different bacteria, scientists simply have to sequence the 16S rDNA genes in a sample. There’s no analogous gene common to all viruses, though.
Because of these technical challenges, surveys of our viral makeup — our virome — are in their early days. But we can already say two things. First: just as every person has their own bacterial profile, so to do they probably have their own viral one as well. Second: most of of the viruses we carry probably want nothing to do with us; they want our bacteria. Williams writes:
Some viruses infect human cells, cause diseases, and then disappear; others leave lingering genetic signatures in the cells; and some can infect cells without causing symptoms at all. But many viruses found in the human body aren’t hosted by our cells. Instead, they’re renting rooms from the body’s bacteria. “The great majority of viruses in the human body are going to be viruses that infect bacteria,” says microbiologist Frederic Bushman of the University of Pennsylvania School of Medicine, Philadelphia. “And many of those are probably impacting humans by influencing the bacteria’s functioning or abundance.”
Even our DNA isn’t entirely our own. Rather, it holds the remnants of viruses that infected our evolutionary ancestors, added themselves to our chromosomes, and stayed put. Over time, many of the As, Cs, Ts and Gs of these viral invaders have been whittled away such that only small bits and pieces remain scattered throughout our genome.
Science journalist Carl Zimmer explains:
Viruses have insinuated themselves into the genome of our ancestors for hundreds of millions of years. They typically have gotten there by infecting eggs or sperm, inserting their own DNA into ours. There are 100,000 known fragments of viruses in the human genome, making up over 8% of our DNA. Most of this virus DNA has been hit by so many mutations that it’s nothing but baggage our species carries along from one generation to the next. Yet there are some viral genes that still make proteins in our bodies.
Most of the scraps of viral DNA are silent and are doomed to remain so. Some, though, are able to turn on at certain times and perform remarkably powerful functions, like turn on genes that help conjoined egg and sperm become an embryo.
When you look in the mirror, what you see reflected looks like whole, single organism: you. But look closer — microscopically closer — and you’ll realize that, as Stanford microbiologist David Relman put it, you’re more like coral , “an assemblage of life-forms living together.”
And that’s an amazing thing.