The natto bacillus is a rod-shaped bacillus that resembles a baton with rounded ends. The size of each bacterium is 2-3 µm (1µm = 1/1000 of a mm). The baton-shaped bacillus stretches as far as it can and then splits as if being ripped apart, after which the process is repeated.
One bacterium becomes two, two become four, and so on. Each propagation takes only 30 minutes and doubles the quantity. Over the course of a single night, the number is increased by a factor of one million.

It should be noted that this is what happens in an environment that is favorable to natto.
This is the proliferation potential when nutrients derived from the boiled soybeans, moisture and a suitable temperature are all present.
However, if the environment is inhospitable to the bacillus...
... then the special ability of Bacillus subtilis, of which the natto bacillus is a member, is triggered. Each bacterium creates a tiny spore known as an endospore that is enclosed in a hard shell, and this endospore halts growth, sending the bacterium into hibernation until the conditions are more favorable. The hard shell is ultimately dissolved, but in its spore state, the bacillus does not die no matter how harsh the environment is. Even if there is no oxygen, or if it is heated for 30 minutes at a temperature of 100°C, or if it is placed in an alkaline or acidic solution, the spore does not die. This is why the natto bacillus is said to be tough.