Out of interest, if humans ever did colonise another planet with living material already on it how would we deal with the alien bacteria and what would be the actual level of threat of it to us, and maybe even vice versa.

In the deepest reaches of the Earth’s oceans there are deep sea vents that spew out clouds of superheated water laced with volcanic chemicals and gasses.

Bacteria can be found at these vents that can be found no where else in the world. Bacteria that thrive on hydrogen sulfide gas poisonous to humans.

Any bacterial level life found on another planet would most likely be as foreign to us as these deep sea bacteria found right here at home.

We do not catch most animal diseases because bacteria (and viruses) are specialised to attack certain hosts. It is possible that we could visit a highly diseased alien environment and have no ill effects whatsoever. However, given time, such things which have very short lives can adapt and would eventually evolve to affect us.

Microscopic life can live in boiling acids (160.C at pH 2), in vacuums, under strong radiation and pressures as well as in various gases. Also very deep underground as well as being inert for millions of years. Despite our efforts, our own bacteria would contaminate an alien environment, and possibly harm life there given time.

As past colonists took rats along with them, possibly future colonists will unwittingly take insects along with them. An alien lifeform in a stable environment can upset the balance, as rabbits did in Australia.