How do physicians chose antibiotics that inhibit the growth of bad bacteria without killing good bacteria?

There are many. Antibiotics run in clifications. Gram-positive covers the penicillin groups, gram negative- the tetracycline, and e-mycin can do both. Then there are the anti-fungals too for the popular fungus. Most and the stronger come in IV forms but still can fall into the same groupings too. The worse the infection, the stronger the antibiotic. Add the many pharmacy textbooks and then the doctor has his/her favorites and your antibiotics are prescribed. Last stop is the pharmacy, and an important one too. Please make sure you use the same one every time. This is so that when or if the doctor makes any mistake with misdiagnosing or the different side effects that might not be good, the pharmacist's computer will automatically warn you, and then the doctor can be called to change it. Good luck and God Bless