To quote:

Catching a free ride to Mars takes more than sticking out a thumb, but some hardy Earth bacteria could survive as hitchhikers clinging to the outside of spacecraft, studies have shown.

Now a set of experiments going up with space shuttle Atlantis to the International Space Station will test how exposure to the harshness of space might change bacteria during a simulated Mars mission.

...

The current shuttle experiment—a collaborative effort between the University of Florida, NASA, the European Space Agency and the German Aerospace Center (DLR) in Cologne—will take place for more than a year on an external space station platform called EXPOSE. That platform will be installed outside of ESA's Columbus laboratory module upon delivery by space shuttle Atlantis flight STS-122.

Full article here:

http://www.space.com/scienceastronomy/071211-st-space-microbes.html