Urine itself has antimicrobial properties which are enhanced when the urine is concentrated. Dilute urine is more likely to support bacterial growth. The renal cortex is more resistant to infection than the renal medulla because there is less blood flow to medulla which leads to less antibody, less complement, and fewer leukocytes. High osmolality inhibits migration and phagocytosis by leukocytes. High ammonia concentration inhibits complement.
Renal infections develop in the medulla.
Catheterization can cause UTI by causing physical damage to the mucosa or by allowing bacterial migration through, around or introduced by the catheter. Repeated intermittent catheterization is associated with less risk of infection than indwelling catheters. If an indwelling catheter is necessary, the catheter and tubing should be a closed system. Physical examination should include palpation of the bladder for wall thickness, degree of distension, masses, calculi, and ease of expression. The kidneys should be palpated for size, contour, and pain.