Bacteria from Earth can easily colonize Mars

Research into bacterial colonization on Mars was not part of the plan to terraform the alien world ahead of human occupation. Instead, three teams investigated how to prevent microbes from Earth from hitching a ride to the red planet aboard spacecraft. It is nearly impossible to remove all biological contaminants from equipment headed to other planets.
By better understanding what organisms can survive in space or on the surfaces of other worlds, mission planners can learn which forms of microscopic life to concentrate on during the sanitation process.
"If you are able to reduce the numbers to acceptable levels, a proxy for cleanliness, the assumption is that the life forms will not survive under harsh space conditions," Kasthuri Venkateswaran of the Jet Propulsion Laboratory and co-author of all three papers, said.
Researchers investigated the problem using independent experiments. The first used the EXPOSE-E facility aboard the ISS. The team then proceeded to expose organisms known to be hardy on Earth to 18 months in space.
"It was found that some… are also partially resistant to the even more hostile environment of outer space, including high vacuum, temperature fluctuation, the full spectrum of extraterrestrial solar electromagnetic radiation, and cosmic ionizing radiation," a group of international researchers wrote.
A second team, composed of researchers from the German Aerospace Center, California Institute of Technology and Jet Propulsion Laboratory exposed bacteria to space conditions, also for a year and a half. They used the European Technology Exposure Facility (EuTEF) on the space station to conduct their experiment.