The twisting helices of DNA within our bodies influence everything from our height to our personality to risk of diseases. Now, it’s clear that our genes also shape our microbiomes—the trillions of microbes that live within us.

By studying 416 pairs of British twins, Julia Goodrich and colleagues from Cornell University have identified the gut microbes whose presence is most strongly affected by our genes. And chief among them was a mysterious bacterium called Christensenella minuta, the one and only member of a family that was discovered just three years agodiscovered just three years ago.

Genetically and physically, it’s rather mundane. It’s yet another rod-shaped, oxygen-hating, nutrient-fermenting bacterium from the Firmicute dynasty—one of the two major groups in