NASA takes a lot of time to kill bacteria and other contaminants on spacecraft that it sends into space. It's particularly important to kill off any bacteria that might be on a spacecraft that will land on the surface of another planet to prevent contamination. The problem NASA is having is that some of the bacteria on spacecraft are nearly impossible to kill.
In fact, some have proven so resilient that NASA is looking for new ways to kill bacteria on future spacecraft. Some of those bacterial spores can survive space and NASA fears that it might be sending life out into space away from Earth. This poses problems for future missions that may send probes to other world's.
One specific bacterium called Bacillus pumilus SAFR-032 has a high resistance to the techniques used to clean spacecraft such as peroxide treatment and UV radiation. Those spores can also survive in the vacuum of space. NASA placed these spores outside the ISS and they survived for 18 months. One of the big challenges for NASA is to develop methods of killing bacteria on smaller spacecraft that can't survive the heat of NASAs currently approved dry heat microbial reduction. Some believe that in the near term this is a moot point because bacterial exchange between mars and Earth has been going on for millennia.