A 65 year old male diabetic presenting with high grade intermittent fever diagnosed to have acute Pyelonenephritis. Inspite of prlonged antibiotics agianst ESBL E coli the fever has relapsed three times and patient has gone into urosepsis in the last 3 months. CT KUB and MRI pelvis are negative for obstrucive uropathy.

No growth on repeated Blood Cultures but urine CS showed E coli ESBL . No evidence of emphysematous pyelonephritis on CT scan. What could be the cause of relapse?