What is a Urinary Tract Infection?

Urinary tract infection is one of the most common bacterial infections, it can be defined as the presence of pathogenic microorganisms in the urinary tract. It is a frequent cause of morbidity and mortality, and a major driver of antibiotic resistance as antimicrobial drugs are often empirically prescribed.

The occurrence of urinary tract infection varies in dependence of age and gender, as well as socioeconomic background. In the United States, this condition is responsible for more than eight million physician office visits every year, and in the United Kingdom, it accounts for 1-3% of consultations in the general practice.

Causes and pathogenesis

The urinary system is composed of the kidneys, ureters, urinary bladder and the urethra, which in most circumstances act in concert to eliminate harmful bacteria. Furthermore, high urine osmolarity and acidity inhibit the growth of a large number of pathogens that can be found in urine.

Nevertheless, bacteria can infect the urinary tract and cause the disease by two main routes: ascending spread of fecal flora and hematogenous route. Migration of organisms from the perineum via the urethra to the bladder (and subsequently to the kidney) is by far the most common route of infection.

Conversely, only around 5% of cases stem from hematogenous route in patients who have bacteremia from other foci of infection such as endocarditis. The pathogenic microorganisms that enter the bloodstream can infect the renal parenchyma, resulting in pyelonephritis and even renal abscesses.

Most common cause of both uncomplicated and complicated urinary tract infections is Escherichia coli, which uses a panoply of virulence factors to cling to epithelial cells and cause inflammation. Other frequently encountered urinary pathogens are Klebsiella pneumoniae, Enterococcus faecalis, Proteus mirabilis and Staphylococcus saprophyticus.

Risk factors

The incidence of urinary tract infections is increased in sexually active women, poor hygiene or instrumentation. Even though physical and immunological host defenses usually destroy pathogenic bacteria before they can establish an infection, impairment of these mechanisms can predispose an individual to infection.

Furthermore, structural and functional abnormalities (such as cysts and diverticula) can disrupt the normal flow of urine and lead to urinary stasis. Such conditions are perfect for bacteria which can thrive and divide more easily, leading to a greater bacterial load and increased likelihood of epithelial adhesion.

Urinary stones, catheters and other foreign bodies can act as a nidus of infection where microorganisms can settle and form biofilms (i.e. whole communities of microbial cells). Such an environment is fruitful for selection of resistance mutations, which can render antibiotic therapy ineffective or only partially effective.

Changes of epithelium in urinary bladder occur during pregnancy, which is also a significant predisposing factor. Organ transplantation and the use of immunosuppressive drugs increase the risk of urinary tract infection and urosepsis in those groups of patients.

Classification of urinary tract infections

Urinary tract infections are generally classified according to their anatomical location, or regarding their severity and complexity. Considering the former, lower urinary tract infections represent an umbrella term for cystitis and urethritis, whereas upper urinary tract infection or pyelonephritis is an invasive infection of the renal parenchyma.

Uncomplicated urinary tract infections are defined as lower tract infections affecting women with no structural, immunological or metabolic predispositions. On the other hand, complicated urinary tract infections involve the upper urinary tract and/or occur in patients with some of the aforementioned predisposing factors.

It must be noted that a large number of women suffer from recurrent urinary tract infections. Recurrences can be divided into “relapses” when symptoms recur on cessation of treatment and the same putative organism is responsible for the disease, and “reinfections” where a new causative microorganism is isolated. In such cases, non-pharmacological methods to reduce recurrences should be pursued, such as cranberry juice and improved hygiene.


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  6. Chiu C. Definitions, Classifications, and Antibiotics. In: Rané A, Dasgupta R, editors. Urinary Tract Infection: Clinical Perspectives on Urinary Tract Infection. Springer-Verlag London, 2013; pp. 1-10.

Further Reading

  • All Urinary Tract Infection Content
  • Urinary Tract Infection Symptoms
  • Urinary Tract Infection Diagnosis
  • Urinary Tract Infection Treatment
  • Urinary Tract Infection Prevention

Last Updated: Aug 23, 2018

Written by

Dr. Tomislav Meštrović

Dr. Tomislav Meštrović is a medical doctor (MD) with a Ph.D. in biomedical and health sciences, specialist in the field of clinical microbiology, and an Assistant Professor at Croatia's youngest university – University North. In addition to his interest in clinical, research and lecturing activities, his immense passion for medical writing and scientific communication goes back to his student days. He enjoys contributing back to the community. In his spare time, Tomislav is a movie buff and an avid traveler.

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