It is the action of the natto bacillus that converts soybeans into natto through the process of fermentation. The natto bacillus which plays the leading role in fermentation is one of the members of a type of microorganism known as Bacillus subtilis. There are said to be around three million varieties of microorganisms in the world, of various types and with various properties. These include bacteria, fungi, protozoa, algae and so on. ("Basics of Fermentation" Hiroshi Tachi, Seibundo Shinkosha Publishing Co., Ltd.)

Bacillus subtilis is a type of bacteria. The subspecies that produces a natto-like aromatic fragrance and fine threads — the natto bacillus — is named Bacillus subtilis var. natto.

Bacteria of the Bacillus subtilis family are found everywhere: in soil, in the air and so on. In these ecosystems, they decompose animals and plants and return them to the soil and air.

Scientific name
Bacillus subtilis
Natto bacillus: Bacillus subtilis var. natto

"Decomposition" or breaking down is a key concept for the creation of natto.
The natto bacillus breaks down the proteins, lipids and other components of the soybean by secreting an enzyme of its own creation, and propagates as it incorporates these components. Vitamins, aromatic substances, fine threads and so on are produced as byproducts of the decomposition process. The components of the soybeans are also converted into a more easily digestible form.

The natto bacillus performs this series of actions as part of its own vital activities. We refer to this process as "fermentation" and are fortunate to receive its benefits.