The secret is out: exhibition explores the trillions of microscopic organisms that live on--and in--us.

• Position: Science & Technology - "The Secret World Inside You" exhibition

HOW DO your interactions with microbes--from the type of environment where you grew up to the number of times you have taken antibiotics, which destroy both bad and good bacteria--influence your health? In what ways can your microbiome be said to be its own organ? Is it possible that the state of the bacteria in your gut plays a role in your mental health? The exhibition "The Secret World Inside You"--using larger-than-life models, computers, videos, art installations, and a live theater--dives into these questions through an interactive tour of the human body, making stops at places where microbes thrive.

The microbes in and on your body are more numerous than the stars in our galaxy. Bunch them into a single unit and they weigh about three pounds--the same as your brain. Moreover, all microbes--organisms too small to be seen with the naked eye--are not disease-causing germs. In fact, most of those living in your body are important factors in ensuring that your digestive and immune systems and your brain are functioning properly.

The human body plays host to trillions of microbes. These include bacteria, viruses, fungi, and other organisms--known collectively as the human microbiome. Microbial genes outnumber the genes in human DNA by more than 100 to one. Indeed, your body contains more microbial cells than human cells.

"We've long known that we are hosts to bacteria on and within our bodies but, historically, microbiologists have focused on studying individual species one at a time," says Susan Perkins, co-curator of "The Secret World Inside You," as well as a curator in the American Museum of Natural History's Division of Invertebrate Zoology and the Sackler Institute for Comparative Genomics. "The increasing availability of advanced research technology like DNA sequencing has given us a different perspective: we see these microbes as part of larger communities that have evolved intimate relationships with us over many millions of years."

Skin. Your skin covers about 20 square feet, making it the largest organ in your body, and microscopic organisms cover it from head to toe. From the point of view of a microbe, your skin is like an enormous continent, with resources that vary dramatically from one region to another.

Differences in skin temperature, texture, thickness, humidity, and chemistry help determine which kinds of microbes live where. The blend of bacteria on your skin also depends on who you are and how you live. For instance, women's hands often have more lactic acid bacteria--similar to those used to make yogurt, pickles, and wine--than men, whose hands tend to have more Corynebacterium cells, related to the bacteria that are used to make the flavor ingredient monosodium glutamate, or MSG. However, even on just one person, left and right hands likely have a different microbial mix because conditions are different on each hand: one hand may be saltier, oilier, or sweatier than the other, and your right and left hands typically may touch different things, picking up different microbes along the way.

New research...