UTI in dogs

Question
Hi. I have a female Jack Russell Terrier who is 5 yrs old. In the last 9 months she has had 5 or 6 urinary tract infections. I have taken her to the vet each time and gone through many tests. They did numerous urine screens and cultures along with xrays to check for stones. None of these have resulted in finding the cause. The antibiotics work for a while but the UTI keeps returning. My vet had me monitor her drinking habits and put her on a cranberry extract. I also was giving her distilled water for a while just in case there was something in the tap water. She has always ate NutroMax dog food. She used to eat the Minichunk but has been on the one for overweight dogs for the last few months due to a knee problem. The vet wanted her to lose weight to help her with her joints. I have been giving her a little Purina chicken & rice canned food before bed to mix in the cranberry extract. I have been mixing it with the dry food and watering it down to get her to take in more water. The vet also had me give her some c/d canned food during the last UTI but there was a concern of how fattening it was. After all this the UTI has returned. Do you think that it is possible the NutroMax may be causing the infections and is there any other food or diet you would recommend to help prevent further infections and still keep her at a healthy weight. Thanks for your help

Answer
I have so little experience here maybe I shouldn't have answered.  I can say, my one puppy never had another UTI after switching to dry Science Diet C/D.  I also adjusted the amount to keep her nice and lean.  

Most dogs will thrive on most foods.  I doubt NutroMax is a problem, but some dogs need a diet that produces lower pH urine.  I have had over a dozen dogs that did well one the Pro Plan I had to switch away  from for the one dog.