Natto is a traditional Japanese breakfast food of whole soybeans fermented with the bacterium Bacillus subtilis var. natto into sticky, glistening beans bound together by long, gooey, spider-web threads. It is fiercely divisive: pungent and ammoniacal, with a nutty, faintly bitter, deeply savoury taste and a slimy, stringy mouthfeel that pulls into silken strands as you lift it. Stirred until foamy and spooned over hot rice with mustard, soy sauce and chopped spring onion — sometimes a raw egg — it is one of the most nutritious everyday foods in the Japanese diet, prized as a source of protein, fibre and vitamin K2.