If the human body contains about 100 trillion cells, how many of these are human cells? All of them?

Not quite. Not even close.

Only 10 trillion, or about 10 percent, are human cells. The rest are microbes. That’s right: 90 percent of the human body is made up of microorganisms.

That was the teaser for the lecture I attended last night by Noah Fierer, a “microbial ecologist” at the University of Colorado. (Gotta love how biology and ecology are no longer separated fields.) Dr. Fierer said microbes, including bacteria, “give us traits we wouldn’t otherwise have.” They digest our food, they regulate immune system functioning, they produce vitamins.

Microbes provide a genetic diversity we don’t have by ourselves. If the human genome includes, say, 20,000 genes, the microbial community living in and on each of us brings a thousand times more genes to the table. The benefits of such diversity are obvious, including more flexible responses to the environment, such as disease resistance, and doing all those jobs we can’t do by ourselves.

In short, each of us is an ecosystem. A unique ecosystem. Each person, said Fierer, has our own resident community. Each of us shares only about 13 percent of our microbial community members with others, including the people we live with. Different bacteria live in different parts of the body. The gut and mouth, of course, include the most—and the most diversity—but different communities reside in different parts of the skin. Your fingertips host a different community than your palms. The sole of your foot is similar to your armpit.

The spot between your eyebrows is incredibly rich in microbial diversity—like the Amazon rainforest of the face—while your cheeks are relative deserts, with little diversity.

When you wash your hands, you cut down the numbers of bacteria on your skin, but your own unique resident community will repopulate within a few hours.

Researchers are learning about microbes because advances in DNA sequencing have taken microbial biology out of the petri dish. It was always a severely limited medium for studying microbes because so few bacteria grow in artificial conditions. Biologists now think, said Fierer, that this is because bacteria feed off the molecules produced by their neighbors. Microbes cannot exist separately, only in community.

And they talk. Boy, do they talk.

Microbiologist Bonnie Bassler has been on the forefront of deciphering bacterial language, which takes place through secreting molecules, like enzymes, which are picked up by their neighbors. Each bacterium secretes both a molecule that recognizes members of its own kind, and a molecule that is like a bacterial universal translator—what she calls the “bacterial Esperanto.” Here is an utterly fascinating 18 minutes of Bassler at TED Talks:

Recent revelations about microbes are revolutionizing views of antibiotics—already a problematic treatment because their effectiveness is waning. Fierer mentioned a number of times that taking antibiotics can open the body to other, new infections. For instance,

You can get skin infections if you take antibiotics because you’re destroying the resident community. It’s like clearing the ground. You clear a piece of land and it’s wide open to whatever wafts in on the breeze.

The idea of the body as an ecosystem could revolutionize our medical paradigm, says Valerie Brown. Instead of seeing the body as an isolated unit fighting off pathogens, the body could be seen as a community of organisms functioning in balance. If this is true, says Brown,

It may be possible to tune that system and prevent many diseases—from acute infections to chronic debilitating conditions—and even to foster mental health, through bacteria.

Medicine could turn away from declaring war on pathogens and instead turn toward fostering the health of the resident community of microbes within each person. One of my favorite lines from Brown’s article:

Medical treatment could regularly involve kindness to microbes.

The revolution in how microbes and bacteria are viewed has led to one of the more astonishing medical developments: treating certain gastrointestinal diseases by transplanting healthy microbial communities of the gut. (Careful, there’s an ick factor: these are fecal transplants.)

Mental health treatments may be revolutionized as well. Brown explains:

For one thing, bacteria produce some of the same types of neurotransmitters that regulate the function of the human brain. The human intestine contains a network of neurons, and the gut network routinely communicates with the brain. Gut bacteria affect that communication.

Bacteria as sophisticated, communicating, neurotransmitter-emitting microbial communities. It’s a far cry from what we used to think of as pond scum. According to Bonnie Bassler, microorganisms developed the original rules for living in community and communicating with one another.

At long last, we humans are developing enough humility to listen.

Thanks to Susan Northleaf, whom I met at last night’s lecture, for alerting me to the Bonnie Bassler video.

FOR MORE INFORMATION:

• Tel-Aviv University physicist Eshel Ben-Jacob researches the foundation of cognition in bacteria and illustrates it with amazing color-enhanced photos of the marvelous shapes bacterial colonies organize themselves into when stressed. (Thanks to Jessica Snyder Sachs’s blog for the tip.)
• See Noah Fierer’s lab and a 2-minute video of Fierer explaining possible forensic uses of hand bacteria.