we have met the microbes, and they are us

Quick question. What percentage of the cells you carry around belong to bacteria?

If you said anything less than 90%, it's time to revisit your preconceptions about the human organism [sub. req.]

The average human is more microbe than mammal, a veritable super-organism comprising 10 times as many microbial cells as human cells. The total number of microbial genes in the human body is thought to outnumber human genes by up to 1000 to 1. Read this article out loud, and long before you reach the end you will have released 10,000 bacteria-laden droplets into the air. With so many microscopic hangers-on you can afford to shed a few. But you would be in big trouble without any at all. In fact, you wouldn't be human, a paradox that scientists are trying hard to get their heads around.

Within five years, geneticists should map the majority of the "microbiome," which is mostly composed of a stable population of microbes--the same "bacterial core" or "scaffold." Variances in the population can be a clue to a person's ancestry, so evolutionary biologists are keenly interested in the results. We already know about the problems bacteria can cause us--acne, gum disease, ulcers, impetigo, among other maladies--and we're getting tantalizing clues that bacteria are implicated in obesity, and might be a part of its solution. More troubling, overuse of antibiotics, which is already implicated in the rise of multiresistant "superbugs," might also be increasing rates of "esophageal disease, asthma, eczema, and allergic rhinitis."

If you're grossed out by the prospect of being an oversized bacterial playground, don't worry: your human cells outweigh the microbes, and your brain is, at least when healthy, mostly microbe-free. But thank the 2.2 pounds of gut bacteria that help you "digest certain foods, metabolise drugs, detoxify noxious compounds, or make essential vitamins." Without them, you'd be only human--and probably dead.