In my efforts to improve and scale up my natto making process, I found I had many questions about the biology of my magical ingredient and probiotic partners in this venture, Bacillus subtilis (B. subtilis).
As a geek at heart, I find that obstacles are often overcome by understanding more about the science underlying what I am doing. So I went to consult Dr. Jonathan Dworkin, a professor and microbiologist at Columbia University Medical Center in Washington Heights. Our chat provided many interesting and useful insights into how these bacteria live, and I want to share some of them with you.
Jonathan's lab and many others around the world have spent years dissecting the nature of signals and events that allow B. subtilis (and other spore-forming bacteria) to emerge from a state of dormancy to reinitiate growth and reproduction (1,2). This is as close as biology (currently) comes to resurrection from death.
B. subtilis is among a specialized group of microorganisms which can undergo a process called sporulation, which means that they can enter into a kind of non-living hibernation state by transforming into an inert, highly indestructible structure called a spore. Sporulation occurs as a protective response to nutrient limitation and probably other yet unidentified environmental stresses (2).
Spores are incredibly resistant to extreme temperatures, desiccation, radiation & chemical insults and can "survive" for indefinite periods of time. When environmental conditions become favorable again, spores can miraculously emerge from this dormant state and "come back to life" (2,3).
New York Natto is made by inoculating steamed soybeans with B. subtilis in spore form. Biologically, the critical step in creating the natto is in germinating or "waking up" the spores to begin living, growing and nourishing themselves on nutrients provided by the soybeans. Exposing spores to a pulse of high heat is believed by many scientists in the field to promote spore germination (2), and this is exactly what happens when spores are rapidly mixed with steaming hot soybeans during natto preparation.
Most non-sporulating microbes are killed by extreme temperatures. This is the basis of sterilization techniques like pasteurization which use the heat of boiling/steaming water to eliminate microbial contamination.
Nature fortuitously allows B. subtilis spores not only to survive, but thrive in response to boiling water temperature, so natto beans are effectively sterilized and simultaneously seeded only with viable B. subtilis. A brilliant marriage of biology and food safety.
References: Dworkin lab website- http://www.microbiology.columbia.edu/faculty/dworkin.html (2) personal communication from Jonathan Dworkin, PhD (3) Wikipedia [sporulation in Bacillus subtilis] http://en.wikipedia.org/wiki/Sporulation_in_Bacillus_subtilis (4) Wikipedia [Louis Pasteur] (5) http://www.devbio.biology.gatech.edu/?page_id=15.