What Causes Urinary Tract Infection?

A urinary tract infection is caused by microorganisms that either ascend from the urethra or that spread to the kidneys from the bloodstream. It is usually caused by the bacteria Escherichia coli, but may also be caused by Staphylococcus saprophyticus, Klebsiella species, Proteus mirabilis, Staphylococcus aurous, Pseudomonas aeruginosa, or other virulent organisms.