Urinary tract infection

1. What is a urinary tract infection (UTI)?

UTI is the presence of microorganisms in the urine or tissues of the normally sterile genitourinary tract. Infection may be localized to the bladder alone or involve the kidneys and, in men, the prostate. Acute uncomplicated UTI occurs in women with a normal genitourinary tract and usually manifests as acute cystitis (bladder infection or lower tract infection). These same women experience, less frequently, kidney (upper tract or kidney parenchymal) infection, referred to as acute uncomplicated or acute non-obstructive pyelonephritis. Complicated UTI occurs in individuals with structural or functional abnormalities of the genitourinary tract, including those with indwelling devices, such as urethral catheters. Recurrent urinary infection may be reinfection, with a new organism, or relapse, when the same organism is isolated posttherapy.

2. What is asymptomatic bacteriuria?

Asymptomatic bacteriuria is isolation of bacteria from the urine culture in quantitative counts consistent with infection but with no genitourinary signs or symptoms attributable to infection.

3. Who gets UTIs?

UTI occurs primarily in two groups of individuals.

• The first group is healthy girls and women with normal genitourinary tracts. About 10% of all women experience at least one episode of infection in a given year, and 20% to 45% of young women will have a recurrence within 1 year following a first infection.

• The second group at risk for UTI are individuals with underlying functional or structural abnormalities of the genitourinary tract, such as obstruction, diverticula, vesicoureteral reflux, or indwelling devices. This group includes infants and most men who experience UTIs, as well as women.

4. What is the pathogenesis of UTI?

Urinary infection usually occurs following colonization of the periurethral area by organisms from the normal gut flora and subsequent ascension of these bacteria into the bladder. This is facilitated by sexual intercourse in women, by urologic procedures including catheter insertion, or by turbulent urine flow in men with prostate hypertrophy. Vesicoureteral reflux, if present, facilitates the ascension of organisms from the bladder to the kidneys. Rarely, UTI follows hematogenous spread from another source, and may present as kidney abscesses. Indwelling urethral catheters and other urologic devices uniformly acquire a surface biofilm composed of microorganisms growing within an extracellular mucopolysaccharide material that they produce. This biofilm incorporates urine components, such as magnesium and calcium ions, and Tamm Horsfall protein. Organisms persist and multiply within the protected environment provided by this biofilm, where there is restricted diffusion of antibiotics and impaired access of host defenses, such as neutrophils.

5. What risk factors are associated with increased frequency of UTI?

Healthy women and girls have both genetic and behavioral risks for UTI. Genetic variables include polymorphisms of genes for the innate immune response and or being a nonsecretor of the blood group substances so bacteria may adhere more avidly to mucosal surfaces. For premenopausal women, 75% to 90% of episodes of infection are attributable to sexual intercourse. Other risk factors include use of spermicides, which disrupt the normal vaginal flora and promote colonization by potential uropathogens, or a new sexual partner within 1 year, which is associated with colonization with new organisms.

A wide variety of functional and structural genitourinary abnormalities contribute to complicated urinary infection, through promoting increased access of bacteria to the bladder or by interfering with normal voiding to allow organisms to persist in the urine ( Table 46.1 ). For patients with indwelling devices, the major risk factor for infection is the duration the device remains in situ.

6. Which are the common infecting organisms in UTI?

Escherichia coli is isolated in 80% to 85% of episodes of acute cystitis and 85% to 90% of episodes of acute uncomplicated pyelonephritis. Uropathogenic E. coli are characterized by the expression of diverse virulence factors including adhesins, iron sequestration systems, and toxins. Pyelonephritis is consistently associated with expression of the P pilus, a Gal-∝(1-4), Gal-β disaccharide galabiose adhesin. Staphylococcus saprophyticus, a coagulase negative staphylococcus, is isolated from 5% to 10% of cystitis episodes. It is virtually only identified as a pathogen in acute cystitis.

A greater variety of organisms are isolated from complicated urinary infection. E. coli remains an important pathogen, but Klebsiella pneumoniae, Citrobacter spp., Proteus mirabilis, Pseudomonas aeruginosa, Enterococcus spp., and other bacteria or yeast are also isolated. These bacteria are more likely to be resistant to antimicrobials. This is attributed to prior antimicrobial exposure or the acquisition of health care–associated organisms following urologic interventions. Urease-producing organisms, such as P. mirabilis, Morganella morganii, and Providencia stuartii , are common in the biofilm on indwelling devices.

7. What are the usual symptoms of UTI?

Bladder infection presents with one or more symptoms of acute dysuria, frequency, urgency, stranguria, hematuria, and suprapubic discomfort. Women with recurrent cystitis can reliably self-diagnose a UTI in more than 90% of cases. Kidney infection presents with costovertebral angle pain or tenderness with or without fever, which is frequently also accompanied by lower tract symptoms. Patients with complicated UTI may present with symptoms of either bladder or kidney infection. Urinary infection in infants is more common in boys and presents as fever and failure to thrive. Patients with an indwelling urethral catheter also usually present with fever without localizing genitourinary findings, although hematuria, catheter obstruction, or costovertebral angle pain and tenderness may be present. Acute prostatitis is a severe systemic illness characterized by high fever, bacteremia, and, often, acute urinary obstruction. Chronic bacterial prostatitis may present as relapsing acute cystitis in older men.

8. How is a laboratory diagnosis of UTI made?

A urine culture confirms the diagnosis and identifies the specific infecting organism and susceptibilities. A urine specimen for culture should be obtained prior to initiating antimicrobial therapy for all presentations of symptomatic urinary infection, with the exception of women with acute uncomplicated cystitis in whom the characteristic clinical presentation is reliable for diagnosis. However, these women should have a urine culture obtained if presenting symptoms are not characteristic, or when there is failure to respond to antibiotics or the recurrence of infection within 1 month of antimicrobial therapy. A voided urine specimen, collected using a method to minimize contamination, is usually appropriate. If a voided specimen cannot be obtained, an in-and-out catheter specimen is recommended.

Patients with a short-term indwelling urinary catheter should have the urine specimen obtained by puncture through the catheter port. When a long-term indwelling catheter is present, the catheter should be replaced and a specimen collected through the replacement catheter to obtain a sample of bladder urine not contaminated by microorganisms present in the biofilm.