Washington ? Eeeww. There’s a zoo full of critters living on your skin — a bacterial zoo, that is. Consider your underarm a rain forest.

Healthy skin is home to a much wider variety of bacteria than scientists ever knew, says the first big census of our co-inhabitants.

And that’s not a bad thing, said genetics specialist Julia Segre of the National Institutes of Health, who led the research.

Sure they make your sneakers stinky, “but they also keep your skin moist and make sure if you get a wound that (dangerous) bacteria don’t enter your bloodstream,” she said. “We take a lot for granted in terms of how much they contribute to our health.”

People’s bodies are ecosystems, believed home to trillions of bacteria, fungi and other microbes that naturally coexist in the skin, the digestive tract and other spots. But scientists don’t have a good grasp of which microbes live where, much less which are helpful, even indispensable, in maintaining health. The NIH’s “Human Microbiome Project” aims to change that, recruiting healthy volunteers to learn what microbes they harbor so scientists can compare the healthy with diseases of microbes gone awry — from acute infections to mysterious conditions like psoriasis or irritable bowel syndrome.

The skin research, published in today’s edition of the journal Science, is part of that project. Scientists decoded the genes of 112,000 bacteria in samples taken from a mere 20 spots on the skin of 10 people. Those numbers translated into roughly 1,000 strains, or species, of bacteria, Segre said, hundreds more than ever have been found on skin largely because the project used newer genetic techniques to locate them.

Topography matters, a lot, the researchers reported. If a moist, hairy underarm is like a rain forest, the dry inside of the forearm is a desert. They harbor distinctly different bacteria suited to those distinctly different environments. In fact, the bacteria under two unrelated people’s underarms are more similar than the bacteria that lives on one person’s underarm and forearm.

Mom’s advice to wash behind your ears notwithstanding, that spot contained the least diverse bacteria — 19 species on average. The most diverse spot: the forearm, which averaged 44 species.

How many are supposed to live there? That’s not clear yet. Some certainly could be tourists, picked up as we go about our day. When researchers re-checked five of these volunteers a few months later, the bacteria in some spots — the moist nostril and groin, for example — proved pretty stable while other spots, including the forearm, had changed quite a bit.

Which are good bugs, and which bad? That depends. A common skin bacteria is Staph epidermidis, found all over the body. Segre said it helps protect us from its nasty cousin, Staph aureus, which about a third of people are thought to carry on the skin or in their nose even if they have no active infection.

But, back to topography, Staph epidermidis itself can harm if it gets under the skin; it’s a common trigger of catheter-caused infections.

The research helps lay the groundwork for what doctors really want to know: What’s different in the skin of people with diseases such as eczema or psoriasis? Those studies are about to begin, says Dr. Martin Blaser of New York University Langone Medical Center, who is leading one on psoriasis and performed some first-step studies of skin bacteria that helped lead to the NIH’s census.

Then there’s the scrubbing question, society’s antibacterial obsession.

“There’s an all-out assault on our normal skin organisms,” Blaser noted. “In trying to get rid of the bad guys, are we getting rid of the good guys?”