[DigDugDude]

I'm a registered nurse, and while not an expert in microbio, I have more training in it than the average bear.

It doesn't take much for bacteria or fungi to start growing on previously filtered water once it's open to air. Just take a glass of filtered water and leave it out for a few weeks. Some glasses won't grow anything. Others will have all sorts of things floating on the surface.

Pasteur is credited for formalizing this knowledge when he figured out that if you used a crooked necked flask to store media, it reduces the chance of airborne contamination.

Once bacteria form a biofilm, it could make it more difficult for any chlorine or UV light to disinfect or sanitize the surface. The bacteria have formed a sort of protective bubble.

Mycobacteria are particularly good at resisting various antibiotics because they have a thicker than average cell envelope. Mycobacterium tuberculosis is another bug in this family.