We rely on commensal bacteria in our gut to help us digest food that our own enzymes can’t breakdown. Yet, when those bacteria escape the intestine, they are associated with chronic inflammatory diseases, including Crohn’s disease and inflammatory bowel disease.

Recently, David Artis, PhD, associate professor of Microbiology, and colleagues reported that a particular type of immune cell is responsible for confining a type of commensal bacteria to the gut. When Artis’ team eliminated these immune cells, called innate lymphoid cells, from mice, one species of bacteria, Alcaligenes, spread beyond the intestine. And suddenly, the immune system, which had ignored the bacteria when they lived in the gut, regarded the bugs as invaders and mounted an attack. In the mice, the ill-placed Alcaligenes were enough to trigger chronic inflammation.

When Artis and colleagues looked for correlates in patients, they found that individual’s with Crohn’s disease and progressive hepatitis C virus infection – both characterized by inflammation – had antibodies against Alcaligenes-specific proteins. It’s not yet clear if the presence of Alcaligenes outside the intestine actually cause the chronic conditions or if they’re just exacerbating an existing problem.

In a contrasting piece of work, though, Artis’ group found that a healthy gut microbiome reduced the likelihood of developing asthma-like symptoms in animal models. When the team treated mice with broad-spectrum antibiotics –– the kind that obliterate many different kinds of bacteria –– the animals were subsequently more likely to develop an allergy-like response to dust mite proteins, which are common human allergens. The same was true if the researchers reared the mice in sterile conditions that prevent them from gaining their normal complement of gut bacteria.

The link between commensal bacteria and allergies may lead to new ways to treat or prevent asthma and allergies in humans, according to Artis. In a more general sense, Artis says the findings make clear that our microbiome has functions and importance not previously appreciated.

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