Microscopic Stowaways on the ISS

A 5000x scanning electron microscope image of E. Coli bacteria. It is a normal resident of human intestines and provides vitamin K and some of the B vitamins. Image Credit: The Centre for Microscopy and Microanalysis, The University of Queensland, Australia.

Microbes are everywhere.

"Just stand and breathe, and you're releasing microbes," Roman said. "You can wash and scrub and use antiseptic soap, and you'll still have microbes on your skin. You have them everywhere: in your clothes, on your skin, in your hair, in your body -- everywhere you could think of..

Many people may find the thought of microbes living on and in their bodies disturbing, but living with an entourage of stow-away microbes is natural.

"Generally speaking, microbes are invisible, and so people just don't think of them as much as you do some other things," said Dr. Duane Pierson, director of microbiology at NASA's Johnson Space Center.

"People need to be reminded that we live in a microbial world," Pierson said. "They were here before us and they'll probably be here afterwards. We co-exist with them very well..."

In fact, bacteria in people's intestines help to digest food, providing some otherwise unattainable nutrients, such as vitamin K. A person's resident microbes also actually protect them from infection by competing with dangerous microbes looking for a place to grow.

While it is natural for a person to live with a host of resident microbes, seven people -- each with their own set of microbes -- living in a small, air-tight can for months or years is certainly not.

"When the crew goes up to the station, they'll each have their own microbial flora, and when they return back, for the most part they've exchanged that flora with each other," Roman said. Most of these exchanged microbes are fought off by the crew's immune systems and their own resident microbes, Pierson noted, but the potential for infection is there.

The first step in protecting the health of the crew is testing each crewmate for infection before launch. Only healthy crew members are allowed to fly into space, and they're quarantined before launch to prevent them from contracting harmful germs at the last moment.