A closed system. Why is this even a surprise?
Didn’t Mir have some sort of “growth”?
It’s not a closed system. They have the vacuum of space for removal.
It isn't a closed system, the ISS is in constant contact with things from Earth. Every flight to the ISS contains bacteria, spores, etc., that we don't even know about or control. Our own bodies are colonies of species of bacteria. We also have shipped plants and animals up there in experiments and for food, which can also carry bacteria.
How to clean a space station might be the most important thing we learn at the ISS.
There are a few things to use:
- As another poster said, coating surfaces with silver would help.
- Bathing modules in UV light would help.
- Ammonia or bleach, followed by venting and replacing a module's atmosphere might help.
- Abandoning the ISS temporarily to send it up above the radiation protection of Earth and let it get bathed in radiation might help.
- Bring along the natural predators of the bacteria to keep it in check. (Design your space station as a terrarium, not a sterile laboratory. Accept that living things will be there not under your complete control.) Only keep laboratory modules sterile.
Any technique would not work on every bacteria, as many species can live in extreme environments.