Foreign body. 1) bacteria located at ou just within urinary opening get pushed up into the bladder, and can occur every time catheter is introduced. 2) everyone with an indwelling catheter will eventually get a uti, takes longer if on antibiotics, but no bladder is immune. Bacteria work their way up between catheter and urethral lining. 3) catheter can be seed for stone formation, also damages bladder lining.
For multiple reasons. Once a urinary catheter is placed, bacterial colonization begins as bacteria spread from the skin along the tube. Within 14 days, 100% of patients with a catheter will have a positive urine culture. This should not be treated unless symtomatic to prevent resistance. Good hygiene, proper maintenance, and early removal of the catheter will lessen the risk of uti.