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.
That’s why studies have repeatedly shown that the bacteria in probiotics are more like tourists than tenants—they pass through without settling down. “You’re trying to establish organisms in an ecosystem to which they haven’t evolved,” says Jens Walter, from the University of Alberta. “They don’t possess the adaptations to be successful.”
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.