Some chalked it up to the effects of exercise, or cleaner air. But over time, it became clear that microbes, ferried into agrarian homes on a conveyor belt of grime, were the real heroes. Early exposures to these farm microbes, scientists reasoned, might teach kids’ immune systems not to overreact to foreign substances—like the ones that trigger asthma attacks—later in life.
This rural revelation was compelling, but acting on it seemed impractical. Most of the initial observations had been made exclusively in the countryside, and, barring a mass metropolitan exodus, researchers worried the forecast for urban children looked bleak.
Now, the findings of an international team of scientists may offer a glimmer of hope for city-dwelling families. According to new research published today in the journal Nature Medicine, microbes in urban schmutz also have the potential to ward off asthma in children—as long as they resemble those found on farms.
The study wasn’t designed to show cause and effect, but it strongly suggests that, while farms are a common backdrop for asthma avoidance, the microbes themselves matter more than the local environment. The findings may even have the potential to inform the development of asthma-preventive treatments, and could someday help safeguard children in rural and urban homes alike.
“This is a very cool, informative study that really advances what we know about the connections between asthma risk and farm living,” says Marie-Claire Arrieta, a microbiologist at the University of Calgary who was not involved in the study. “It shows it’s not the farm, per se, but the bacteria that are behind this...and gives us hope that someday, we may even be able to take farm bugs to the city.”
Rural and urban environments typically harbor different microbial species. While farms run thick with single-celled critters that hail from soil and animals, city living rooms tend to be dominated by bacteria that associate with human bodies.
But within these broad categories, there’s still plenty of variation. To see if metropolitan microbes could mimic those found on a farm, a team of researchers led by Pirkka Kirjavainen, a microbiologist at Finland’s National Institute for Health and Welfare, sampled living rooms in nearly 400 rural and suburban residences around Finland. Each was home to at least one young child whom the researchers monitored through early development.
When the infants were two months old—an age, Kirjavainan explains, when babies are still developing their immune systems and are likely inhaling and ingesting a lot of microscopic household debris—the researchers visited their homes and collected samples of living room dust. By genetically analyzing the single-celled life within, the team was able to identify a microbial signature unique to farms. They then used their model as an index to measure how “farmlike” the collective microbes in each residence were. The higher a home scored, the less likely its resident infant was to receive a diagnosis of asthma by the age of six.
Without an explicit intervention, these kinds of studies can’t show definitive proof that farmlike microbes are actually armoring kids against asthma, Kirjavainan says. But a pattern that manifests over the course of six years, he says, “supports [the notion of] causal effects.”
To see if the findings held true in another population, Kirjavainen and his colleagues repeated their analysis in a cohort of more than 1,000 children in Germany. This second dataset, compiled by a different group of researchers, was composed of dust samples from mattresses in the homes of kids who had already developed asthma.
In spite of these differences, however, the researchers were able to use the Finnish farm microbe template to zero in on residences with high and low asthma risk. This, Kirjavainen says, shows the model can be applied to multiple geographical locations and groups of people, and can even accommodate different types of residential grit.
“At this early stage [of research], it’s important that the phenomenon isn’t necessarily limited to just one environment or population,” says Ariangela Kozik, a microbiologist at the University of Michigan who was not involved in the study. However, she adds, microbial differences between Finland and Germany did exist, and adjustments will need to be made to accommodate other environments on a more global scale.
On the whole, the research isn’t ready to be deployed as a diagnostic. But in the future, these kinds of microbial headcounts could come in handy for identifying homes that are more protective, says Charisse Petersen, a microbiologist at the University of British Columbia who was not involved in the study.
The findings also lend support to a longstanding theory called the hygiene hypothesis, which posits that humankind’s zeal for cleanliness has stripped us of the microbes that help shape and rein in immune responses in early childhood. In recent years, however, many researchers have begun to put a finer point on the idea: Health isn’t just a question of dirty or clean—and not just any microbes will do. Rather, which species are present may make all the difference. Studies like this get us a little closer to identifying our most important microbial allies, Arrieta says.
Such developments are particularly timely as asthma rates continue to increase worldwide. A cure for the condition remains elusive, and many available therapies remain imperfect, Petersen says. “At this point, the future is all about prevention.”
To this end, researchers may someday be able to leverage farm microbes to manufacture something akin to a dust probiotic, possibly in the form of nasal spray, or a treatment to coat urban living rooms in common rural species.
Before that happens, though, researchers will need a better understanding of what’s going on at the molecular level, Kozik says. Once the microbes enter the human body, it’s not entirely clear how they work their magic—and though several studies in mice have provided some hints, many questions remain about how effective interventions would be in people.
In the meantime, there may still be a few actions urbanites can take to recreate protection in their daily lives, Petersen says. For instance, in the study, urban dwellings where people wore shoes indoors were more likely to harbor farmlike microbes. Gardening or owning pets might not hurt either, she adds. More work is needed to understand these connections, she says, but these findings and others suggest it can’t hurt to loosen our standards about letting the outdoors in.
“It’s not just, ‘You don’t live on a farm, you’re out of luck,’” she says. “If you take small steps to replicate a farm environment, you might get protection later in life. It gives us hope that we can take small steps to protect our kids.”