In that post I wrote that "The average surface temperature on Mars is -63° C (-81° F). Rarely, at the equator, temperatures at the very surface reach 20° C (68° F), but even then the temperatures just a few inches above that are sub-arctic. Average barometric pressure on earth is 1013 millibars. Average barometric pressure on Mars is about 6 millibars, which is less than the inside of an early vacuum tube. Martian atmosphere is 95% carbon dioxide, with 210 ppm water vapor. Earth's atmosphere is 78% nitrogen and 21% oxygen, with about 25,000 ppm water vapor at the surface." All of this is true. The implication was that any organism which lived comfortably inside a human being could not possibly survive in that environment.
I was wrong. Meet Deinococcus radiodurans.
It is highly resistant to radiation, dehydration, heat, cold, vacuum, and acid. It can survive being nuked so well that scientists have experimented with encoding information into is DNA to survive a nuclear holocaust (starting, ironically, with Disney's song It's a Small World). In other words, it would survive just fine on the surface of Mars.
Oh, and it lives in our poop.
Polyextremophiles such as D radiodurans could in fact cross-contaminate between terrestrial and martian ecosystems. And this, Houston, could be a problem.