I had gone through your question and understand your concerns. The most common cause is secondary immunodeficiency due renal transplantation. Gram negative bacteria account for 70% of UTI and, Escherichia coli is the most often clinical isolate in solid organ transplant recipients. The incidence of Pseudomonas aeruginosa infection tends to be higher than in the general population, especially in the case of pyelonephritis. Some reports have documented an increase in the prevalence of Pseudomonas aeruginosa, coagulase-negative Staphylococci and Enterobacter cloacae in the first 3-5 weeks after transplantation. Kidneys and intestines have common blood supplier and infection from intestine can go to kidneys. The another thing is resistance of the most urinary pathogens to antibiotics, that s why infection can develop despite antibiotic protection. Hope this answers your question. If you have additional questions or follow up questions then please do not hesitate in writing to us. I will be happy to answer your questions. Wishing you and your mother good health.