Let them eat dirt

The University of British Columbia’s Brett Finlay demonstrated that microbes might protect children against asthma, more evidence that exposure to bacteria keeps us healthy
Tyler Irving
The University of British Columbia
British Columbia
Molecular Biology
The soles of a child’s blue rubber boots, caked in mud

About half the cells in your body aren’t yours — they belong to the bacteria and other microorganisms that colonize everything from your mouth to your large intestine. If that thought grosses you out, consider this: growing evidence suggests that many of those bugs may actually be beneficial to your health.

“A hundred years ago, we realized that microbes caused disease, and we went to war with them,” says University of British Columbia microbiologist B. Brett Finlay. Improved hygiene, combined with the advent of antibiotics, greatly reduced the rates of infectious diseases. But Finlay thinks that things may have gone too far. “As we’re going after these harmful microbes, we’re also getting rid of the normal microbes that we should be exposed to,” he says.

The diverse mix of microbes living inside you — known as your microbiota — is as unique as your fingerprint.  Studies from Finlay’s lab and others have shown that when the composition of this microbial community shifts one way or the other, it can have a significant effect on your health.

For example, antibiotics used to kill infectious bacteria also wipe out many of our native microorganisms, and studies have shown that children who receive antibiotics in the first year of life are about 20 percent more likely to develop asthma. The correlation suggests a link between asthma and the microbiota, but until recently nobody had demonstrated this link experimentally.

Finlay and his team tackled the problem by analyzing microbiota from the feces of more than 300 Canadian children, some who developed asthma and some who didn’t. “We were able to show that four microbial species were much more prevalent in kids who were protected from asthma than kids who were at risk for it,” says Finlay. When germ-free mice were inoculated with microbial mixes containing higher levels of these same four species, they were also protected from developing asthma. The study was published in the journal Science Translational Medicine.

Studies by other groups have suggested microbial links with disorders ranging from diabetes and obesity to depression, anxiety and even autism. Finlay and his former postdoctoral fellow Marie-Claire Arrieta recently published a book, Let Them Eat Dirt, which summarizes the scientific evidence for these connections. It also contains practical advice about how to ensure that kids get enough beneficial bacterial exposure without putting themselves at risk for disease. “It’s fine if kids lick the floor of their own home, but maybe not the floor of a crowded subway station,” says Finlay. He also recommends against the use of antimicrobial hand sanitizers, which studies have shown to be no more effective than soap and water.

In the future, Finlay expects that a number of therapies will involve manipulating microbiota in some way. “There’s some good stuff coming, but it’s just not fully developed yet,” he says. “What we’re rediscovering is that we as a species have evolved with these microbes, and when we get rid of them, we are getting rid of a key piece of ourselves.”