Hello there, I am dr.milan an infectious disease specialist answering your question. I feel that better you consult an urologist. You can consult nephrologist also no problem in that also. Now e.coli is most common infecting organism in urinary tract. Problem is right that bladder is weakened and prostate is operate so normal urinating mechanism is compromised now so it is the reason why recurrent infection is happening. Everytime you go for culture & sensitivity test and we need to follow sensitivity pattern. Long term uses of one antibiotic can cause resistance so we need to change antibiotic according to report. But you need to be vigilant when there is infection showing by increase in frequency, burning, pain, fever or any other symptoms. Hope i have given appropriate guidance to you. if you have any query you can consult me anytime. Give me star rating according to your satisfactory level. Thanking you.