It's hard to know every nuance of a person't situation from a post, but here are my thoughts on it...
There's always "the course" that has a chance to bring your GPA down a bit. At some schools it's the weed-out Gen Chem. At others it's Biochemistry. For you, it may be this Micro. You have a very nice buffer with a 3.85 GPA, and I can't imagine that you'll do so poorly that it will bring your GPA down any considerable amount. Micro is 3-5 credits out of 120+. That's why it's always important to do your best and bring up your GPA where you can, because there's always going to be a class or two that's not going to go your way. And Adcoms know this.
IMO, taking all your other classes for a grade, and taking Micro as a Pass/Fail, may draw attention to the fact that you made that conscious choice, that you weren't confident in your ability. Remember, "Pass" is C and above. When an Adcom sees "Pass" is this what they think of? With your academic track record thus far, and I don't know you other than from your post, I feel that you should take the course for a grade and you'll be just fine. Best of luck!