Probiotic yogurt and other "gut friendly" foods line supermarket shelves. But what's the best way to feed the microbiome within us that we need to survive—and what exactly is a microbiome anyway? Scientific American contributing editor Katherine Courage investigates these questions in this deep dive into the ancient and artisanal foods that feed a healthy gut, as well as the biological science that's catching up to these traditions. Courage takes us to mountain villages in Greece, cheese caves in Switzerland, and modern biotech labs to explain what's going on inside when we eat foods like kimchi, sauerkraut, Gruyere and Emmental cheese, slow-fermented olives, kombucha, and of course, yogurt.