Bacteria (Salmonella typhimurium) flown in space on the shuttle were shown to be more virulent than the control strains held on Earth.
What I found pretty neat was something that one of the researchers said: "Wherever humans go, microbes go; you can't sterilise humans. Wherever we go, under the oceans or orbiting the Earth, the microbes go with us, and it's important that we understand... how they're going to change."
This is a good reminder that microbes, the prokaryotes but also the protists, play extremely important ecological roles in the environment and within other organisms, that we frequently overlook because of their small size. Concepts which seem 'exotic', such as parasitism, 'alternative' metabolisms, and even perhaps the ecology of soil bacteria, are actually more commonplace than we think.
In terms of sheer number and biomass, the microbes are the rule, not the exception, and they're everywhere. Even in space flight, the pinnacle of human technological achievement, we've still not figured out how to deal with the 'problem' of microbial hitch-hikers.
Perhaps we shouldn't think of them as problems to be eliminated. After all, they've been with us since the beginning and have had a hand in our own evolution (for example vitamins produced by gut bacteria which our own metabolisms cannot produce). They've pretty much become part of us, and attempting to eliminate them would be a sign of hubris and only put our own selves in the way of illness.
PNAS article link.