One point about bacteria that seems to be missed in all talk about extraterrestrial colonisation: in recent times more and more is being discovered about animals' (including humans) need of microbes (bacteria, viruses, fungi). The gut biome is important, and has a huge impact on us. Microbes are important in many other ways, most of which we probably do not know.

So when we venture off earth, we must take at least some microbes with us; we don't know which, and may not even know they exist. I'm not concerned here with us contaminating our destination (we will have to take plants and animals with us to support us anyway); it must of course be discussed and taken into account. But what happens if microbes which are essential to us die out? Certainly the diversity of microbes in a small group of people will at the least be less than on Earth, which is probably a bad thing.

Microbial supplement shipments from Earth and the maintenance of some sort of culture lab will need to be a thing for any future space colonisation effort. If you were born on Earth it will probably never matter, but the point of a colony is that one day people will be born there, and never been exposed to Earth germs.

The fight to keep a functional immune system for those separated from Earth for generations will become an increasing challenge. A common cold could become a fatal pandemic.

But by the time such things are possible, medical technologies will also have advanced to levels we haven't yet imagined, and dealing with a weak immune system could be as trivial as popping a pill by then. In the short term radiation levels are a much more upfront risk to health. Mars is the equivalent radiation dose of a CT scan every two weeks of time spent there. Not so terrible for a short term visit but a significant increase on ordinary exposure levels.

As for life on Mars, I'm 90% certain it's stone cold dead and always has been. If it's not we'll know as a result of ExoMars and Mars 2020.

Multicellular life evolved on a planet occupied for 2.5 billion years by microbial life. Land could not have been colonised without a symbiotic relationship between the progenitors of plants and funghi. We are 30% microbe and cannot survive outside of this complex ecosystem. ( )