I read Apollo 12 returned with a camera fro a Moon probe which apparently carried S. mitis to the Moon, survived under those conditions, and was viable after returning to Earth. How did NASA/CDC decide the S. mitis was not a lab contaminant.
It seems probable from recent research that the viable microbes were indeed contaminants. The story of living microbes retrieved by the Apollo 12 astronauts from a previous Surveyor lander has been told many times and has been widely accepted as evidence for the ability of some microbes to survive space exposure. New information, however, suggests that these were contaminants probably introduced on the trip back to Earth from the Moon.
NLSI Interim Director
July 18, 2006