“What is the strongest antibiotic for kidney infection?”

Antibiotics do not really, and different strengths. They either match the sensitivities of the bacteria involved in an infection or they do not. Maybe one of the best antibiotics for urine stuff used to be used to be Levaquin, but because it was use so much it is starting to be resisted by some bacteria. This happens because of anabiotic overuse. Your doctor should be aware of what the right anabiotic profile in your area should be for various infections. Good luck

The antibiotic that is used is determined by the bacteria the urine culture identified. If the decision to treat is before a culture comes back the the bacteria suspected based on male/female, pregnant nonpregnant, old/ young , will determine which antibiotic is used