nattō (Japanese: 納豆 hiragana なっとう) is a traditional Japanese food made with fermented soybeans. Soybeans are cooked and then exposed to Bacillus subtilis var. natto.[1]​ This fermented food stands out for the growth of the bacteria, which develops a kind of viscous slime (made up of a biofilm), and because the seeds inoculated by this bacteria give off a strong characteristic odor. nattō is a traditional breakfast served in northern Japan, seasoned with mustard, chopped leek and shoyu ('soy sauce') and accompanied by white rice.[2]​.

This fermented food contains vitamins, enzymes as well as essential amino acids, but perhaps its main virtue is an extract of it, called nattokinase, which, as mentioned in scientific studies in its country of origin, acts as a powerful anticoagulant agent capable of dissolving blood clots (thrombi). It is necessary to mention that, due to this property, the intake of this food should be consulted with the doctor, in the case of being under treatment with usual anticoagulant medications, due to the possible interaction that could occur.[3]​.

For its preparation, the smallest and white or yellow soybeans are selected. They are steamed and allowed to cool until they are warm (approx. 45 °C), and then the bacteria Bacillus subtilis var. natto", which are obtained in specialized stores, which ferment the grains. They are then kept in fermentation chambers in certain conditions of temperature and humidity for about 20 hours. Afterwards, this mixture is matured in refrigeration for another 24 hours, after which a type of paste is obtained, which is what is known as nattō.[4][5]​.