With NASA's space shuttle program officially at an end, the agency is making preparations to benefit the future of spaceflight, which includes ambitious plans for long-duration human missions to Mars or an asteroid.

But to make these big missions happen, researchers are thinking small. So small, in fact, that they're focusing on the microscopic bacteria in our guts.

This is because keeping humans alive and healthy on years-long missions is a major challenge, complicated by the unknown effects of microgravity on various nasty bugs that make humans sick. At least one common pathogen, Salmonella, gets more virulent in space, and microgravity alters the activity of other microbes as well.

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