From the article:
But for the general public, the implications of the study are simpler. “A microwave is not a pure, pristine place,” Porcar says. It’s also not a pathogenic reservoir to be feared, he says. But he does recommend cleaning your kitchen microwave often — just as often as you would scrub your kitchen surfaces to eliminate potential bacteria.
An interesting read, one of the reasons why I clean the things that I own. Microorganisms will find ways to survive and thrive on any surface. I like to first clean and then use a natural sanitizer after.