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

Number of providers by state who wrote at least 3,000 prescriptions for Schedule 2 controlled substances in 2012 in Medicare Part D. ProPublica/NPR hide caption

itoggle caption ProPublica/NPR