It can be easy to forget but bacteria is omnipresent in everywhere human can ever be, and even beyond.

It is not possible to scrub every trace of bacteria if the goal is ever to have a living human there. The human itself is the carrier of germs. Remove all the germs from the human and that human will not be very happy or very healthy for long.

Not to mention it is ridiculously hard to fully sterilize everything, especially in places where lots of nooks and tiny crinks exist like a space station. All it take is one bacteria and you will have a thriving and diverse ecosystem in a decade or two, complete with niche subspecies and unique populations[1]

[1] https://en.wikipedia.org/wiki/E._coli_long-term_evolution_ex...