The large bowel is not sterile in healthy adults. However, when large bowel bacteria colonize the bladder, the result is cystitis or bladder infection. Cystitis, though rare in young men, can occur as a result of urethral obstruction from prior infections of sexually transmitted disease or from congenital defect of the urethra which requires surgical correction. They may occur in both men and women as a complication of a lower UTI or arise without lower urinary tract involvement. The latter may occur in younger women or men as a result of obstruction somewhere in the urinary tract.
Examples are a kidney stone or abdominal tumor which obstructs the ureter.
Symptoms of upper UTI include fever, chills, lower back pain, nausea, and vomiting. The most common bacteria causing the infection is E. coli . Men, though generally not prone to lower UTIs, may develop upper UTIs as they become older and cannot fully empty their bladders as a result of prostatic enlargement. Urine cannot remain sterile when it sits in the bladder for a long period of time. In elderly men this evolves into full-blown urinary retention where a liter or more may have to be drained by catheter.