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Which microorganisms most commonly cause urinary tract infections?

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The most common cause of urinary tract infection is invasion into the urethra of Escherichia coli, a type of bacterium that normally inhabits the bowel. — urinary tract infection The most common cause of urinary tract infection is invasion into the urethra of *Escherichia coli*, a type of bacterium that normally inhabits the bowel.

Urinary tract infections (UTIs) are primarily caused by the invasion of microorganisms, most commonly bacteria, into the urethra and bladder.1

The most frequent cause of a UTI is the bacterium Escherichia coli, which typically inhabits the bowel but causes infection when it enters the urinary system. The second most common bacterial cause is Staphylococcus saprophyticus, a bacterium often found on human skin.12

Other organisms that may be involved in UTIs include:
* Less common bacteria: Proteus mirabilis and species within the genera Klebsiella, Mycoplasma, Enterococcus, Pseudomonas, and Serratia.1
* Fungal organisms: In rare, complicated cases, fungi such as Candida and Coccidioides may be involved.1
* Other pathogens: Infrequent causes include various parasites (such as threadworms and flukes) and viruses, such as herpes simplex virus type 2 (HSV-2).1