I’m going to outline a few steps you can follow before you speak with your employee, while you speak with her, and after you speak with her.

Before we dive in, though, I'm wondering whether it's your employee's position you find negative and undermining, or how she expresses it. I think it's important you clarify this for yourself before you talk with her. It’s one thing to discourage sarcasm or passive-aggressiveness, for example, and quite another to discourage disagreement with management.

I’m going to assume that the problem is the way she disagrees rather than that she does. If this is true, her behavior may be the result of not feeling safe or not having the skills. The good news is you have an opportunity to make it clear you welcome differing viewpoints and demonstrate how to express them.