One bad grade is not the end. I am sure adcoms can overlook a B- or, god forbid, a C, if the rest of your academic record is excellent. I got quite a few B's on my prereqs, including a B- in one quarter of physics and a B- in one quarter of Ochem. I still got into 5 schools. Don't worry. If you have a good OVERALL application, it doesn't matter. Good luck.