NASA proved in the 70s that some microorganisms survive atmospheric rentry. In fact, they could not possibly be killed by any means known to man, except for some enormous cosmic event with very powerful radiation / cosmic rays. This was done to question the idea of it being "safe" or not to travel in space or other planets, if it is possible to "scrub" our ships clean and avoid bringing germs with us. Apparently thats not 100% possible, some critters survive space and the harshness of re-entry just fine.