Natto is fermented soybeans with Bacillus subtilis, know as Bacillus natto. It is a Japanese natural probiotic food. We simply eat them on top of steamed rice. It is also popular as the filling of hand-rolled sushi (temaki zushi). Natto is usually mixed with soy sauce, Japanese mustard (karashi), dried bonito flakes (katsuobushi), and seasoned dried seaweed (nori) and served. In Japan these days, it is popular to put natto on bread and toast it, cook it in miso soup, or mix it with fried rice.