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Eat a germ, fight an allergy

Anybody who's ever picked up an M&M off the sidewalk and popped it into his or her mouth (and, really, who among us hasn't?) will be gratified to learn that the more germs you're exposed to, the less likely you are to get asthma or related allergies such as hay fever and eczema.

At least that may be an emerging scientific consensus, upon publication of a new study in the New England Journal of Medicine, in which investigators compared the microbial environments of kids who grow up on farms to those of kids who grow up in the city or suburbs.

That country kids' mattresses contained more microbes, and more different kinds of them, may not be so astonishing in itself. But in the study, the number and diversity of the microbes in the kids' environment had a strong inverse correlation with their likelihood of having asthma, hay fever or eczema. More microbes, and more diversity among them, meant less wheezing, sneezing, or itching due to these so-called "atopic disorders." (Atopic comes from Greek words meaning, roughly, "out of place," and refers to the immune system's inappropriate response to harmless foreign substances.)

It's been known for some time that farm kids are considerably less likely to become allergic or asthmatic than city kids and that, moreover, the prevalence of atopic disorders has been growing in leaps and bounds worldwide as societies urbanize. But in theory, this could have been due to a number of factors (such as air quality, for example) that have nothing to do with germs. The new study ruled out at least the more obvious of these other factors, and even singled out a few particular microbial species whose presence appeared most protective against asthma.

Notably, the investigators were able to conduct their broad bacterial census by means of a new counting technique that involves quantitative analysis not of the full-fledged bugs themselves, but rather of pooled, purified DNA from the samples. (Each different type of germ has its own distinctive DNA "bar code.") That makes it possible to spot even microbial types that are impossible to grow in culture. Stanford's David Relman, MD, has put this kind of approach to good use in tracking the changes (due, say, to ingestion of antibiotics) in our intestinal ecosystems, wherein dwell about ten times as many microbial cells as there are human cells in our entire bodies.

The new findings lend support to the so-called "hygiene hypothesis": the notion that childrens' immune systems can go haywire in the absence of the constant microbial companions to which we have grown accustomed over millions of years of evolution.

I don't know that those of us who are raising our kids in urban, suburban, or exurban clean rooms should encourage our kids to "stoop and scoop" every time they spot a germy gem on the candy-coated sidewalk. But maybe we should go easy on the Lysol. A little dirt, it seems, can't hurt.

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