Microbiologist Christina Agapakis (left) and artist Sissel Tolass show off the cheese they made with bacteria from human skin. The project was part of Agapakis' graduate thesis at Harvard Medical School. Courtesy of Grow Your Own ... Life After Nature at Science Gallery at Trinity College Dublin hide caption

itoggle caption Courtesy of Grow Your Own ... Life After Nature at Science Gallery at Trinity College Dublin

He's not just getting a cold. He's building his microbiome. iStockphoto.com hide caption

itoggle caption iStockphoto.com

Say hello to your microbiome, Rob Stein. Our intrepid correspondent decided to get his gut bacteria analyzed. Now he's wondering if he needs to eat more garlic and onions. Morgan Walker/NPR hide caption

itoggle caption Morgan Walker/NPR

Knight (left) and Bucheli take soil samples from beneath one of the decomposing bodies. Katie Hayes Luke for NPR hide caption

itoggle caption Katie Hayes Luke for NPR

Billie Iverson, 86, of Cranston, R.I., recently underwent a transplant of intestinal microbes that likely saved her life. Ryan T. Conaty for NPR hide caption

itoggle caption Ryan T. Conaty for NPR

We may not see them, but we need them. iStockphoto.com hide caption

itoggle caption iStockphoto.com

It's busy down there: a gut bacterium splits into two, becoming two new cells. Centre For Infections/Science Photo Library/Corbis hide caption

itoggle caption Centre For Infections/Science Photo Library/Corbis

Streptococcus bacteria, like this strain, can be found in our guts. Janice Haney Carr/CDC Public Health Image Library hide caption

itoggle caption Janice Haney Carr/CDC Public Health Image Library

Bad bug: The bacterium Clostridium difficile kills 14,000 people in the United States each year. Janice Carr/CDC/dapd hide caption

itoggle caption Janice Carr/CDC/dapd