Imagine that you take some North American mice, breed them in captivity for many generations, and then release them in small numbers into a South American jungle. Smart money says that these house-trained creatures wouldn’t last very long. And yet, this is effectively what we’re doing whenever we buy and consume probiotics.

These products—yogurts, drinks, capsules, and more—contain bacteria that supposedly confer all kinds of health benefits. But most of the bacterial strains in probiotics were chosen for historical reasons, because they were easy to grow and manufacture. They aren’t A-listers of the human gut, and they aren’t well-adapted to life inside us. To make things worse, they’ve been effectively domesticated, having been reared in industrial cultures for countless generations. And they’re delivered at very low concentrations, outnumbered by the bacteria that already live inside us by hundreds or thousands of time.