My wife has had several bladder infections during the last 12 months, has taken several courses of antibiotics and has lost weight. bladder infection keeps on returning. what tests are to be done to establish the real cause?

Recurrent UTI: Several issues can cause recurrent urinary tract infection (UTI). Maybe estrogen lack especially if your wife is past menopause, and UTIs sometimes are prevented by vaginal estrogen. Internal problems like bladder diverticulum and kidney or bladder stone can do it. UTIs can be triggered by sex. Low dose antibiotic (3 times a week or after sex) often is helpful. See an infectious disease specialist

Colovesical fistula: Is among hard to diagnose causes for recurrent UTI's that can be otherwise caused by chronic constipation as well. Causes for fistulas may be inflammatory such as in diverticulitis or malignant conditions. What makes it likely is the weight loss that is of concern in this context. Stick to her doc for until cause discovered.