Why don't all these factors affect experiments?

Bacteria and countless microorganisms are all around us, no matter where we are (unless you’re in a super strict containment facility, but otherwise) right? I mean, sure we have aseptic techniques and wash things down, but there’s still things in the air and things that seep in the room, etc.

Then how come scientists are able to do experiments without having results skewered by this fact? I mean, I’ve been in many lab courses and I had to do things with DNA, for example. But I always wondered, since nothing is truly sterile, why nothing happens to the DNA. I also had a neuro class where we had to dissect rat brains to get neurons. How come we’re still able to visualize the neurons in the end being that there’s all this microbiota all around us?

I feel stupid asking this but. . . whatever.