Natto is a Japanese food made from soybeans. When you mix it, long sticky threads appear. What are they? They are mostly long chains of glutamic acid. This is the same thing that gives kombu its umami taste.

The chains are not in the soybeans at first. Natto bacteria make them during fermentation. Scientists also found that a small "jumping" piece of DNA can stop the bacteria from making the threads.

When you stir natto, it produces long, stretchy threads. These threads are made mainly of poly-gamma-glutamic acid. It is a chain of thousands of units of glutamic acid, the substance behind the umami taste of kombu.

The long chains tangle together and hold water, which creates the sticky texture. The chains are not in the soybeans at the start. Natto bacteria produce them during fermentation. In 2010, Japanese researchers read the entire genome of natto bacteria.

In 2011, another Japanese team found that a short "jumping" piece of DNA sometimes moves into a gene called swrA. When this happens, the bacteria can no longer make the sticky chains. So natto's stickiness depends on stable bacterial genes.

Stir a pack of natto and it pulls into long, stringy threads. These are composed mainly of poly-gamma-glutamic acid, a polymer made of thousands of linked units of glutamic acid, the same amino acid responsible for umami in kombu stock. The long molecules entangle and trap water, which gives natto its stretch, and a chain of fructose called fructan is also present.

The polymer does not come from the soybeans. Natto bacteria, relatives of a common soil bacterium, synthesize it during fermentation. When Japanese researchers sequenced the full genome of a natto strain in 2010, they found that it closely resembled laboratory strains but retained gene features that wild strains use to make the polymer, an ability laboratory strains have lost.

In 2011, another Japanese team showed that a mobile genetic element called IS4Bsu1 can spontaneously insert itself into a gene named swrA, leaving the bacteria unable to produce the sticky substance. Restoring swrA brought the ability back. Popular claims about natto's health benefits have little support from human studies so far. One nutrient does matter, though. Natto is rich in vitamin K2, which can weaken the blood thinner warfarin, so patients taking that drug should consult a doctor.