Which bacterial species is the most common cause of urinary tract infections?

Escherichia coli is the most common bacterial cause of urinary tract infections. Usually shortened to E. coli, it normally lives harmlessly in the intestines but can enter the urethra and travel upward into the bladder.

Most urinary infections are caused by uropathogenic strains adapted to survive in the urinary tract. Their surface structures help them attach to bladder cells, resisting removal when urine flows. UTIs are especially common in women because the urethra is relatively short and lies close to the anus.

E. coli is not the only cause. Proteus mirabilis, Klebsiella pneumoniae, and some Staphylococcus species can also cause UTIs, particularly in hospitals or people with urinary catheters. Proteus is additionally associated with kidney stones because it can raise urine pH.

The bacterium’s presence in the gut explains why hygiene, sexual activity, and anatomical factors can influence infection risk, but an intestinal E. coli strain is not automatically harmful. The key distinction is between ordinary gut residents and specialized strains that invade the urinary system.