An small microbial ecosystem has formed on the International Space Station
arstechnica.com
arstechnica.com
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...
It's the same for us; people often get sick after moving far away, because they aren't used to the local bacteria and fungi yet. So if we ever intend to, say, travel to Mars, we'd better come up with ideas how we can safely maintain a stable ecosystem on a space station that actively accounts for bacteria, fungi, and probably even plants.