"The typical human is home to a vast array of microbes. If you were to count them, you'd find that microbial cells outnumber your own by a factor of 10. On a cell-by-cell basis, then, you are only 10 percent human. For the rest, you are microbial. (Why don't you see this when you look in the mirror? Because most of the microbes are bacteria, and bacterial cells are generally much smaller than animal cells. They may make up 90 percent of the cells, but they're not 90 percent of your bulk.)

This much has been known for a long time. Yet it's only now, with the revolution in biotechnology, that we're able to do detailed studies of which microbes are there, which genes they have, and what they're doing. We're just at the start, and there are far more questions than answers. But already, the results are astonishing, and the implications profound.

Even on your skin, the diversity of bacteria is prodigious. If you were to have your hands sampled, you'd probably find that each fingertip has a distinct set of residents; your palms probably also differ markedly from each other, each home to more than 150 species, but with fewer than 20 percent of the species the same. And if you're a woman, odds are you'll have more species than the man next to you. Why should this be? So far, no one knows.

But it's the bacteria in the digestive tract, especially the gut, that intrigue me most. Many of these appear to be true symbionts: they have evolved to live in guts and (as far as we know) are not found elsewhere. In providing their habitat — a constant temperature, some protection from hostile lifeforms and regular influxes of food — we are as essential to them as they are to us.

And they definitely are essential to us. Gut bacteria play crucial roles in digesting food and modulating the immune system. They make small molecules that we need in order for our enzymes to work properly. They interact with us, altering which of our genes get turned on and off in cells in the intestinal walls. Some evidence suggests that they are essential for the building of a normal heart. Finally, it seems likely that gut bacteria will turn out to affect appetite, as well as other aspects of our behavior, though no one has shown this yet. (Imagine the plea: I'm sorry, sir, my microbes made me do it.)

Together, your gut microbes provide you with a pool of genes far larger than that found in the human genome. Indeed, the gut "microbiome," as it is known, is thought to contain at least 100 times more genes than the human genome. Moreover, whereas humans are extremely similar to one another at the level of the genome, the microbiome appears to differ markedly from one person to the next. "

Olivia Judson in the NYT