The connection between microbes and lifespan dates back to Elie Metchnikoff—an eccentric Russian Nobel laureate who the microbiologist Paul de Kruif once described as a “hysterical character out of one of Dostoevsky’s novels.” He believed that intestinal microbes produced toxins that caused illness, senility, and aging, and were “the principal cause of the short duration of human life”. (His claim, though baseless, apparently started a fashion for colostomy in the early 20th century.) On the other hand, he also thought that some microbes could prolong life by producing lactic acid, which killed their harmful cousins. That was why, Metchnikoff believed, Bulgarian peasants who regularly drank sour milk would often become centenarians.

In 1908, Metchnikoff wrote about his ideas in a book called The Prolongation of Life: Optimistic Studies—an ironic title given that the man was a profound pessimist who had twice tried to kill himself. Still, he also quite literally put his money where his mouth was by regularly drinking sour milk, and created a fad that would culminate in the modern probiotics industry. Metchnikoff died at the age of 71, and his claims haven’t quite stood the test of time. But more recently, several groups of scientists have shown that animal microbiomes can indeed influence the lifespans of their hosts.

In 2013, Filipe Cabreiro showed that metformin—a drug that’s used to treat type 2 diabetes, and that’s being investigated for anti-aging properties—lengthens the lives of nematode worms, but only if the worms have microbes in their guts. More recently, Dario Valenzano showed that the killifish—an extremely short-lived fish that’s being increasingly used in studies of aging—lives longer if old individuals consume the poop of younger ones, suggesting either that old microbiomes quicken the deaths of these fish, or that young microbiomes can prolong their lives.

Despite these promising hints, it’s hard to work out exactly why and how the microbiome influences the pace of aging, because these communities can be bewilderingly complex. When you have a huge range of microbe species exchanging an even wider range of chemicals, it’s hard to tell which particular bug or molecule is important.

So Wang decided to sweep that complexity aside and focus on a very simple partnership. Her team member Bing Han started with a library of E. coli strains that were each missing a single gene, but were otherwise identical. He then fed these strains to the nematode C. elegans—a small transparent worm that features heavily in aging research, and whose body and genes have been thoroughly characterized.

Of the 4,000 or so E. coli strains, Han found that 29 extended the worms’ lives by at least 10 percent. And 19 of these “also protected the worms from age-associated diseases” like cancer and neurodegenerative conditions, says Wang. “They lived longer and better.”