Japan has adopted many strechy or slimy textured food into its culture, including Nameko mushrooms and Chinese yam. Among them, natto has one of the longest streches which record shows that the threads can hold up to 16.3 meters (53.4 ft).
The strechiness or slime is produced from the Natto bacillus fermenting the soybean. The texture differs depending on the types of soybeans or how the fermentation is managed. It mainly consists of polyglutamic acid and fructan which holds the "umami". This "umami" is the secret to the tasty flavor of natto.

Fine threads are produced when you lift natto