QT bloom.....can bacterial live on PVC parts?

yeah absolutely. Nitrifying bacteria are benthically oriented and almost always live on surfaces off all kinds. I don't know what their chances of survival is when they are removed from the substrate from scraping etc. but usually it's not important. Pretty much nitrifying bacteria will live on any surface, pads, glass, pipes, sand, fish, and so on. Yeah runing a "dirty" filter pad will help. Although diatoms are not necessarily a certain sign that you've had a fundamental change in your bacterial community, however it is possible and diatoms may be indicating that.....that sounds weird, it sounds like I'm saying that diatoms don't prove anything but they do prove it....lol what I mean is not quite paradoxical, it's like that saying: absence of evidence is not evidence of absence.