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A urinary tract infection (UTI) occurs when germs (bacteria) invade the urinary tract. The urinary tract is made up of your kidneys, bladder, ureters, and urethra. The ureters are the tubes connecting the kidneys to the bladder. The urethra is the tube carrying urine from the bladder to outside your body.

According to the National Kidney Foundation, 80 to 90 percent of UTIs are caused by a bacteria called Escherichia coli (E. coli). For the most part, E. coli lives harmlessly in your gut. But it can cause problems if it enters your urinary system, usually from stool that migrates into the urethra.

UTIs are incredibly common. In fact, 6 to 8 million cases are diagnosed each year in the United States. While men aren’t immune, women are 30 times more likely to develop a UTI, mostly because of the design of their urinary tract.

Urine is mostly made up of water, salt, chemicals, and other waste. While researchers used to think of urine as sterile, it’s now known that even a healthy urinary tract can host a variety of bacteria. But one type of bacteria not normally found in the urinary tract is E. coli.

E. coli often gains entry into the urinary tract via stool. Women are particularly at risk for UTIs because their urethra sits close to the anus, where E. coli is present. It’s also shorter than a man’s, giving the bacteria easier access to the bladder, where the majority of UTIs occur, and the rest of the urinary tract.

E. coli can spread to the urinary tract in a variety of ways. Common ways include:

  • Improper wiping after using the bathroom. Wiping back to front can carry E. coli from the anus to the urethra.
  • Sex. The mechanical action of sex can move E. coli-infected stool from the anus into the urethra and up the urinary tract.
  • Birth control. Contraceptives that use spermicides, including diaphragms and spermicidal condoms, can kill the healthy bacteria in your body that protect you from bacteria like E. coli. This bacterial imbalance can make you more susceptible to a UTI.
  • Pregnancy. Hormonal changes during pregnancy can affect the growth of certain bacteria. Some experts also think that the weight of a growing fetus can shift your bladder, making it easier for E. coli to gain access.

UTIs can cause a range of symptoms, including:

  • an urgent, frequent need to pee, often with little urine output
  • bladder fullness
  • burning urination
  • pelvic pain
  • foul-smelling, cloudy urine
  • urine that’s brownish, pink, or tinged with blood

Infections that spread all the way up to the kidneys can be particularly serious. Symptoms include:

  • fever
  • pain in the upper back and side, where the kidneys are located
  • nausea and vomiting

Diagnosing a UTI can involve a two-part process.

Urinalysis

To determine if there’s bacteria in your urine, a doctor will ask you to urinate in a sterile cup. Your urine will then be examined under a microscope for the presence of bacteria.

Urine culture

In some cases, especially if you don’t seem to be improving with treatment or you get recurrent infections, a doctor may send your urine out to a lab to be cultured. This can pinpoint exactly what bacteria is causing the infection and what antibiotic effectively fights it.

The first line of treatment for any bacterial infection is antibiotics.

  • If your urinalysis comes back positive for germs, a doctor will likely prescribe one of several antibiotics that works to kill E. coli, since it’s the most common UTI culprit.
  • If a urine culture finds a different germ is behind your infection, you’ll get switched to an antibiotic that targets that germ.
  • You may also receive a prescription for a drug called pyridium, which helps reduce bladder pain.
  • If you tend to get recurrent UTIs (four or more per year), you may need to be on low-dose antibiotics daily for a few months.
  • Your doctor may also prescribe other medications for treatment that are not antibiotic based.

Treating an antibiotic-resistant UTI

Bacteria are becoming increasingly resistant to antibiotics. Resistance occurs as bacteria naturally change to breakdown or avoid the antibiotics typically used to fight them.

The more exposure a bacterium gets to an antibiotic, the more likely it is to alter itself to survive. Overuse and misuse of antibiotics make the problem worse.

After a positive urinalysis, your doctor might prescribe Bactrim or Cipro, two antibiotics often used to treat UTIs caused by E. coli. If you’re not better after a few doses, the E. coli may be resistant to these drugs.

Your doctor may recommend doing a urine culture in which the E. coli from your sample will be tested against a variety of antibiotics to see which one is most effective in destroying it. You may even be prescribed a combination of antibiotics to fight the resistant bug.

While infection with E. coli accounts for most UTIs, other bacteria can also be the cause. Some that might appear in a urine culture include:

  • Klebsiella pneumoniae
  • Pseudomonas aeruginosa
  • Staphylococcus aureus
  • Enterococcus faecalis (group D streptococci)
  • Streptococcus agalactiae (group B streptococci)

UTIs are some of the most common infections doctors see. Most are caused by E. coli and are successfully treated with a round of antibiotics. If you have symptoms of a UTI, see a doctor.

Most UTIs are uncomplicated and don’t cause any lasting harm to your urinary tract. But UTIs that aren’t treated can progress to the kidneys, where permanent damage can occur.