Lactobacillus acidophilus, which is one variety of the genus Lactobacillus is one of the common active cultures found in yogurt and in the human gut. Scimat Scimat/Getty Images/Photo Researchers hide caption

itoggle caption Scimat Scimat/Getty Images/Photo Researchers

Rob Knight, co-founder of the American Gut Project at the University of Colorado in Boulder, works in the lab where the samples are processed. The American Gut Project hide caption

itoggle caption The American Gut Project

After a quick swipe and online registration, these test tubes were ready to ship back to the lab at the University of Colorado in Boulder for sequencing and analysis. Katherine Harmon Courage for NPR hide caption

itoggle caption Katherine Harmon Courage for NPR

A turkey vulture makes quick work of a dead rabbit at Martin Luther King Jr. Regional Shoreline park in Oakland, Calif. Sebastian Kennerknecht/Minden/Corbis hide caption

itoggle caption Sebastian Kennerknecht/Minden/Corbis

Many artisan cheese producers never pasteurize their milk – it's raw. The milk's natural microbial community is still in there. This microbial festival gives cheese variety and intrigues scientists. iStockphoto hide caption

itoggle caption iStockphoto

We are all Russian nesting dolls: Our intestines house many bacteria, which house many viruses. These so-called bacteriophages are likely as important for our health as the bacteria they live in. Lisa Brown for NPR hide caption

itoggle caption Lisa Brown for NPR

Rugby and meat: a treat for the gut? A study suggests yes. Here Tony Woodcock (left) and Owen Franks of the All Blacks rugby team turn sausages on the barbecue in 2011 in Christchurch, New Zealand. Phil Walter/Getty Images hide caption

itoggle caption Phil Walter/Getty Images

Even some euro bank notes may need a good scrubbing. Like dollar bills, these notes are made from cotton and they harbor an array of bacteria. Thomas Leuthard/The Preiser Project/Flickr hide caption

itoggle caption Thomas Leuthard/The Preiser Project/Flickr