HHMI investigator Ruslan Medzhitov, an immunologist at Yale University School of Medicine, recently discovered an entirely new role for our resident bacteria: They help protect us from radiation and poisons. As he explained in a paper in Cell published July 23, 2004, he came to this conclusion indirectly while studying the cells of the innate immune system, our first line of defense against pathogens. Medzhitov expected that the receptors on these cells would detect only the bad microbes. "We thought these receptors would simply ignore any components of good bacteria," he says. "But on the contrary, they recognized both good and bad." As he later found, it was absolutely essential for these immune-system cells to recognize good bacteria to stimulate the intestinal tissue's systems of maintenance and repair.

In one experiment, Medzhitov's team worked with three groups of mice that were exposed to fairly strong radiation, as much as might be used to kill tumor cells. One group, which had normal bacterial "flora" in their guts, survived the radiation with relative ease. When the team irradiated a second group of mice, whose colons had been deprived of normal bacteria by antibiotics, they all died. Then the researchers tried to prevent such deaths by giving a third group of mice—similarly deprived of normal bacteria—a chance to drink water that had been laced with some components of good bacteria. Although they received the same dose of radiation as the second group during that time (one week), they all survived. "The cells of the mice's innate immune systems recognized the bacterial components they had swallowed," says Medzhitov. "That activated their immune systems and protected the mice from some of the damage they would otherwise have suffered." The team obtained similar results in mice exposed to toxic chemicals.

This finding raises the hope that human cancer patients may be protected in similar ways. Restoring the normal supply of good bacteria in their intestines might help them avoid a possible side effect of radiation or chemotherapy: an injured and bleeding gut, which can be fatal.

—Maya Pines

Photo: Amy Etra

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Reprinted from the HHMI Bulletin,
Winter 2005, pages 26–30.
©2005 Howard Hughes Medical Institute