That’s why probiotics don’t seem to have any effect on the make-up of the microbiome—the community of microbes that lives within us. It’s also why these products have been so medically underwhelming. The most discerning reviews suggest that they are useful for treating some kinds of infectious diarrhea, but little else. And over the last decade, European Union regulators have been so unimpressed by the evidence behind probiotics that they banned every single health claim that appeared on these products’ packaging—including the word “probiotic” itself.

The concept is sound, though. We know that the bacteria in our microbiome are important for our health, and that changes in the microbiome have been linked to many conditions including inflammatory bowel disease, colorectal cancer, diabetes, and more. So it should be possible to improve our health by taking the right microbes. The problem is that we do so in a crude and naïve way. These are living things and we are ecosystems. You can’t just introduce the former into the latter and assume they’ll take hold. You need to know why they might succeed or fail.

That’s what Walters and his team have started to do. They focused on a specific strain of Bifidobacterium longum, which is a common, stable, and dominant part of the human gut. María Maldonado-Gómez, from the University of Nebraska, asked 23 volunteers to take daily doses of either B. longum or a placebo pill, and checked their stool for signs of the strain’s DNA.

In most of the volunteers, the bacterium disappeared within the first month or even the first week. But in a third of them, it persisted, and for more than half a year in some cases. Unlike normal probiotics, this strain seemed to establish a permanent foothold. “I never expected that,” says Walters. “Even with part of our core microbiome, I thought that our resident strains would outcompete the new one.”

In a way, they did. By comparing the volunteers’ microbiomes, Maldonado- Gómez showed that his B. longum strain was less likely to settle down if its new hosts already had B. longum strains of their own. That makes sense: Closely related microbes should be more similar, and thus more likely to compete for the same nutrients, resources, or living spaces. If many kinds of B. longum are already present, there are few niches for an incoming strain to fill.

Maldonado-Gomez also found that the ingested strain was more likely to wash out if a volunteers’ microbiome carried a few dozen particular bacterial genes, the vast majority of which are involved in breaking down carbohydrates and other nutrients. Again, this makes sense: If the native microbes are using these genes to digest whatever food is available, there’s nothing for an immigrant strain to eat.

These results show that it is possible to turn a swallowed microbe into a permanent part of the gut, and they hint at the type of factors that make for successful colonization. “I’m excited,” says Walters. “I think it really does show that we might be able to modulate gut ecosystems, by going in and establishing certain microbes. We didn’t look at health, and we’re still trying to identify what microbiome configurations are associated with disease. But if an individual misses or loses strains that are important for their health, it could be possible to redress that.”