Here’s a case where just the title of an article is awesome: “Pyrosequencing-Derived Bacterial, Archaeal, and Fungal Diversity of Spacecraft Hardware Destined for Mars”. Sadly it’s not open access but the abstract is worth a read at a minimum.
The authors conducted environmental surveys in cleanrooms and of equipment destined for Mars. Basically they found that the rigorous cleaning protocols employed in the cleanrooms were enough to remove most, but not all of the potential interplanetary hitchhikers. They found several taxa that repeatedly survived the cleaning process.
The authors discuss why this is such a problem, as well as noting:
“The majority of bacterial OTU observed as being recurrent belonged to actinobacteria and alpha-proteobacteria, supporting the hypothesis that the measures of cleanliness exerted in SAC [spacecraft assembly cleanroom] may inadvertently select for organisms which may be the most fit to survive long journeys in space.”