Urine Infection or Sterile – Causes, Symptoms, Treatment

Is urine sterile?

Sterile means that there are no bacteria, viruses or other microbes. Normally urine is sterile. It is produced in the kidneys from filtered blood, passes down the ureters and into the bladder for storage. There should be no bacteria, viruses or other microbes anywhere along this path from the point where urine it is produced in the kidneys to where it is stored in the bladder, unless a person has a kidney infection, bladder infection or a systemic infection.

However, when urine is being passed out of the bladder into the environment then it has to pass through the urethra. The outermost (distal) part of the urethra is not sterile. Therefore urine may be tainted with bacteria, viruses or other microbes that are present as it exits the urethra. Even in these instances it is just a few microbes that may then be found in the urine.

What is a urine infection?

The term infection means any invasion, damage and/or destruction of tissue caused by microbes or viruses. Urine is not living tissue and while infectious agents can be in it, this does not mean that the urine is infected. Instead it means that the urine is not sterile or contaminated with bacteria, viruses and other microbes. This can lead to an infection of the living tissue of the kidneys and urinary tract.

This type of infection is more correctly referred to as a urinary tract infection or UTI for short. It is a common problem especially in females who have a shorter urethra than males. Most urinary tract infections arise when microbes, usually bacteria, enter through the urethra to reach to the bladder. The inflammation of the urethra is known as urethritis while the inflammation of the bladder is known as cystitis.

The spreading of the infection from the distal (bottom/outermost) parts of the urinary tract to the upper parts is known as an ascending infection. In most instances the infection stops at the bladder. There are some uncommon cases where this infection can continue up the ureters to the kidneys. However, most of the time a kidney infection is where the microbes reach the kidney through other routes like via the bloodstream.

Causes of Urine Infection

The causes of infectious agents in the urine (which is not always an infection) and the causes of an infection of the urinary tract are discussed below. It is important to differentiate between these two conditions involving the urinary tract as the term urine infection can be misleading.


The presence of bacteria in the urine is not necessarily an indication of a urinary tract infection (UTI). A condition known as asymptomatic bacteriuria can occur where there is a large number of bacteria in the urine without any symptoms. Although bacteria in the urine will also be found with a urinary tract infection (UTI), bacteriuria does not always mean that an infection is present. Therefore if there are no symptoms then bacteriuria may not be treated. However, if a person is at risk of an infection occurring then treatment may be started before the infection arises.

Read more on asymptomatic bacteriuria.

Urinary Tract Infection

Although bacteria may reach the outermost parts of the urethra, it does not usually cause an infection or travel further inwards to the bladder for several reasons. However, sometimes it can infect the urethra and continue to the bladder where it leads to a bladder infection. This is more likely to occur in women who have a shorter urethra than men.

The more common bacteria to cause a urinary tract infection is E.coli. It is abundant in the colon, rectum and anus. The bacteria may be easily passed from the anus to the urethra with fecal particles. Insufficient cleaning after defecation and inappropriate wiping after passing stool (wiping from back to front instead of from front to back) are more likely to spread the E.coli bacteria from the anus to the urethra.

Pregnant women, people who have a weakened immune system like with diabetes, postmenopausal women, using certain types of birth control, recent urinary tract surgery and having abnormalities of the urinary tract are some of the risk factors associated with a urinary tract infection (UTI). Urinary catheterization is another risk factor and is also one of the more common causes of bacteriuria.

Bacteria can also be introduced during sexual activity. Some of these types of urinary tract infections may therefore be sexually transmitted infections caused by various bacteria and not due to E.coli. However, sexual activity also increases the risk of UTIs caused by E.coli. The overall risk of UTIs also increases with multiple partners.

Read more on causes of UTIs.

Signs and Symptoms

Asymptomatic bacteriuria, as its name suggests, does not present with any symptoms. The bacteria in the urine is only discovered with diagnostic testing of the urine. Urinary tract infections (UTIs) on the hand usually present with symptoms although there are instances where mild or even chronic infections can be without noticeable symptoms. The signs and symptoms of urinary tract infections includes:

  • Burning sensation when urinating.
  • Constant urge to urinate.
  • Frequent urination usually of small volumes of urine.
  • Pelvic pain and discomfort usually below the umbilicus (belly button).

These are the main symptoms but there may also be:

  • Dark brown to pink or red color of the urine.
  • Cloudy urine usually due to pus.
  • Offensive odor or strong ammonia smell of the urine.

If the infection spreads higher up to involves the kidneys then there my be additional symptoms like fever, nausea and flank pain.

Treatment of Urine Infection

As previously stated, the presence of bacteria within the urine where there are no symptoms indicative of a urinary tract infection (UTI) may not require any treatment in most people. However, treatment may be prescribed for people who are at a high risk of developing a UTI.

Treatment is necessary for a urinary tract infection and this usually involves antibiotics. Oral antibiotics usually suffice but if the infection is persistent or severe then intravenous (IV) antibiotics may be necessary. Water consumption should also be increased to flush out bacteria from the urinary tract.

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