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For decades, scientists have theorized that kids actually need to get a bit dirty for their immune systems to develop fully.
It’s known as the ‘hygiene hypothesis‘. And, as childhood rates of allergies, asthma, and hay fever rose drastically in industrialized Western nations, scientists noticed the same did not go for children raised on farms.
“Girls and boys who grow up on farms and are exposed to a wider variety of microbes have the problem far less often,” says epidemiologist Markus Ege, of LMU University Hospital and the Institute of Asthma and Allergy Prevention at Helmholtz Munich in Germany.
That phenomenon is known as ‘the farm effect’.
Now, Ege and his colleagues have found exactly which bacteria species mediate the farm effect in rural European children. This discovery goes some way towards verifying the hygiene hypothesis once and for all.
Referring to the hygiene hypothesis, Ege says, “this kind of research can only show more or less convincing correlations.”
“But with our new study, we can make a much stronger case,” he continues, “because we can identify the individual links in the proposed causal chain: the bacteria, the relevant microbial metabolic products, and the human receptors.”
Allergies, asthma, hay fever, and eczema: all of these conditions arise when the body’s immune system unleashes an exaggerated immune response to harmless triggers.
If the hygiene hypothesis really has legs, scientists should be able to find specific microbes that, when humans are exposed to them, somehow give our immune systems the challenge they need to develop a more appropriate response, reducing the incidence of immune conditions.
In the new study, the researchers pointed the finger at nine suspect bacteria species.
“We identified a small number of bacteria, which we can now pinpoint down to the species level, such as Romboutsia timonensis and Glutamicibacter arilaitensis,” explains bioinformatician Giulia Pagani, of Helmholtz Munich.
“These gram-positive bacteria together mediate two-thirds of the entire farm effect for asthma protection and half of the effect for hay fever and atopic eczema.”
The researchers discovered these species in dust samples collected from the mattresses of 1,018 rural South German schoolchildren, and the cow shed air at the farms of 47 of the kids.
The nine bacteria linked to the farm effect made up 6 percent of the relative abundance of microbes in mattress dust, but 25 percent in the cow shed samples.
Kids who were exposed to these microbes, and especially those who lived on farms, were much less likely to develop childhood asthma, in a classic example of the observed farm effect.
These microbes originate from the guts of cows, and then wind up in their poop, which may be how they become suspended in the air in cow sheds.
Farm kids spend time around cows, inhale the microbes, and, as their presence in mattress samples suggests, bring the microbes back into their homes where moderate exposure may continue.
That moderation is an important factor here, given the bacteria produce metabolites known to activate a receptor known as AhR, which triggers immune signaling and gene expression.
“High levels of AhR activation are detrimental, whereas moderate levels, as they may occur by inhalation of bacterial metabolites in cow sheds, are beneficial,” the team writes.
When activated, AhR induces a strong anti-inflammatory response which may help prevent the development of asthma and allergies.
Scientists will need to conduct further experiments to determine the exact mechanisms by which these bacteria mediate the farm effect, since an observational study like this doesn’t show that level of detail, and can’t prove cause.
But the chain of events, from cow to bacteria to air to child’s immune system, is finally coming into focus.
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“Taken together, this analysis describes a strong and consistent association between specific environmental exposures and natural protection from asthma and allergies,” the authors write.
“Broken down to a molecular level, these results may advance understanding of the mechanisms of asthma and allergy protection. Ultimately, this may foster the development of effective and well-tolerable prevention strategies for the most common chronic conditions in childhood.”
The research was published in NEJM Evidence.
This article was fact-checked and edited by Michael Irving. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.
