Friend or Foe? The Colon Knows 'Good' Bacteria From 'Bad'

A scanning electron microscope's view of Escherichia coli bacteria. CREDIT: National Institutes of Health

The human gut is teeming with bacteria, most of which helps us digest food and fend off the bad guys in the belly. But how does the body tell the good from the bad?

New research is pointing to gut-specific white blood cells (called Treg cells), which "learn" to identify and then protect the good gut bacteria, telling our bodies "Don't mess with them."

"Since we've had these microbes living with us for the millennia, we've developed a tolerance to them," said Josef Neu, a researcher from the University of Florida who wasn't involved in the study. "That same tolerance with those Treg cells helps prevent us from getting certain types of diseases, like colitis."

The study, led by Chyi-Song Hsieh, of Washington University School of Medicine, in St. Louis, looked at the white blood cells present in the guts of lab mice. They saw that the gut naturally has its own population of the Treg cells which protect friendly gut bacteria.

These protecting cells must rely on some common factor shared by foreign (yet friendly) gut bacteria as well as our own gut cells. If that's the case, Neu speculated what would happen if the protector cells weren't around when our immune system was developing, saying if those bacteria aren't there, the body could react badly to our normal cells.

"I would go on to speculate that this may be one of the first steps that might help us determine if there are individual types of microbes that we could utilize to help develop tolerance to self," Neu told LiveScience.

For instance, an autoimmune gut disease like Crohn's or celiac disease could be treated by exposing the patient's immune system to the right kind of bacteria. Exposure to these protector microbes might temper the immune system so that it stops attacking similar proteins in the body and destroying its own cells.

A researcher who wasn't involved in the study, Fagarasan Sidonia from the RIKEN Research Center for Allergy and Immunology in Kanagawa Japan, told LiveScience in an email that the study "is an interesting piece of work that raises more questions than answers," but has some reservations about the work.

"I am almost convinced that there are Tregs [the gut-specific white blood cells] induced in the gut," she said. "But what remains unclear is how many in normal circumstances and their functional properties — which are not answered by this paper."

The study will be published in the Sept 22 issue of the journal Nature. Though mice serve as good biological models for humans, the researchers can't be sure the same holds true in us and further testing is necessary.

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