Microscopic Stowaways on the ISS

Fungi come in a wide variety of sizes and forms. Tiny, one-celled yeasts (inset) are important for baking breads and fermenting wines, beers and vinegars. Many medicines are produced with the help of fungi, most notably, the antibiotic, Penicillin. Mushrooms are fungi we buy at the supermarket.

Once on the Space Station, the air, water and surfaces with which the crew members interact must be kept clean.

The air in the Space Station will be kept in constant motion, and all the air on the Station will pass through filters -- called High Efficiency Particle Air (HEPA) filters -- on its way to the temperature and humidity control systems.

"The filters were originally designed to remove particulates," Pierson said. "They're very good at removing small particles," such as microbes.

Microbes can ride in the air on particles of dust or in tiny clumps of bacteria or fungi. On Earth, there might be a couple hundred or thousand microbes in each cubic meter of air.

Water will be disinfected by a machine called a "catalytic oxidator," which heats the water to as much as 265 degrees F. The organic molecules in microbes are oxidized by this process, which kills nearly all of them. Just to be sure, the water is then treated with iodine.

After this disinfection, the water should have less than 100 microbes in 100 milliliters of water.

"The water is extremely clean if you compare it to the water that you drink at home," Roman said. "The water on the Station is many, many times cleaner."

For the health of the crew as well as the Station's hardware, microbes must also be kept from growing on surfaces and in nooks and crannies.

"The biggest threat to the Station from the microbes is degradation of the materials," Roman said. "They'll eat pretty much anything."

"As they grow on surfaces, (fungi) produce an acid which will eventually corrode the material," Roman continued. "They start using most materials as a source of food. Have you seen bathroom tile that's been overgrown by mold? Over time, you will notice that the mold has kind of eaten the tile and grout."