Pathogenesis

Acquisition of urinary infection with catheter use is virtually always through ascending infection (Table 108.2). For indwelling urethral catheters, bacteria usually ascend into the bladder on the biofilm which forms on the external and internal surfaces of the catheter tubing, but may also ascend through reflux of contaminated urine up the drainage tubing. Disruption of the closed drainage system from the bladder to the drainage bag may also introduce bacteria, and there is a high incidence of bacteriuria within 24 hours following such a break in the system. Bacteria introduced at the time of catheterization account for less than 5% of infections.

Bacteriuria is a predictable consequence when an indwelling catheter remains in situ for a long enough time. The likelihood of developing bacteriuria is 3% to 7% per day while the catheter remains in situ. The determinants of symptomatic infection are not well described, but trauma to the mucosa or catheter obstruction may precipitate CA-UTI.

With intermittent catheterization, organisms are repeatedly introduced into the bladder at catheterization. Individuals managed with intermittent catheterization usually have a neurogenic bladder with incomplete bladder emptying so organisms, once introduced, may persist in the bladder. The organisms which cause bacteriuria are usually present as colonizing bacteria in the periurethral area, but may also be introduced by contamination of the catheter or on the hands of the individual performing catheterization.

Bacteriology

Urinary infection identified in the setting of urethral catheterization is considered complicated UTI. Escherichia coli remains an important pathogen in these infections, but other organisms are also frequently isolated. These include other Enterobacteriaceae such as Klebsiella pneumoniae, Citrobacter species, Enterobacter species, and Serratia marcesens. For long-term indwelling catheters, in particular, infections with urease-producing organisms such as Proteus mirabilis, Morganella morganii, or Providencia stuartii are common. Pseudomonas aeruginosa and other gram-negative nonfermenters such as Acinetobacter species may be isolated, as well as gram-positive organisms, particularly enterococci and coagulase-negative staphylococci. Candida albicans and other yeast species also occur, usually isolated from subjects receiving antimicrobials. The high frequency of recurrent infection and repeated courses of antimicrobials promote emergence of more resistant organisms.

Polymicrobial bacteriuria is characteristic of infection in subjects with long-term indwelling catheters but may also occur with other types of catheterization. Long-term indwelling catheters or short-term catheters in situ for more than a few days are covered with a bacterial biofilm, on both interior and external surfaces. This biofilm is composed of microorganisms, bacterial extracellular glycopolysaccharides, and protein and minerals incorporated from the urine. There is a complex microbial flora with multiple organisms growing in the biofilm; usually two to five organisms are present in a mature biofilm. The biofilm is an environment within which microorganisms are relatively protected from both antimicrobials and the host inflammatory and immune response. Urine specimens obtained for culture through the biofilm-laden catheter are contaminated by organisms present in the biofilm which may not be present in bladder urine. The number, type, and quantity of organisms isolated from these specimens may differ from a specimen of simultaneous bladder urine.

Morbidity and mortality

Most catheter-acquired urinary infections are asymptomatic. However, catheterized subjects are at risk for increased morbidity from symptomatic UTI. Pyelonephritis, fever, and bacteremia may require hospitalization or result in extended hospitalization when nosocomially acquired. Local complications including prostatitis and epididymitis, purulent urethritis, urolithiasis, and urethral abscesses may occur. Crystalline biofilm formed by urease-producing organisms, principally P. mirabilis, is the most common cause of catheter obstruction. Acute urinary infection in subjects with spinal cord injury or other neurologic diseases may present as increased lower-limb spasticity or autonomic hyperreflexia. Urinary infection in residents with chronic indwelling urethral catheters is the most frequent cause of bacteremia in long-term care facilities, with these residents experiencing three times the incidence of fever as bacteriuric long-term care facility residents without an indwelling catheter. Occasionally, acute urosepsis occurs, but mortality directly attributable to urinary infection is uncommon relative to the high frequency of bacteriuria.

Diagnosis

A diagnosis of urinary infection in a catheterized patient requires microbiologic confirmation. Clinical findings will then determine whether infection is symptomatic or asymptomatic. Culture of an appropriately collected urine specimen is essential. The specimen must be collected before antimicrobial treatment is initiated. It may be obtained directly at the time catheterization is initiated, by intermittent catheterization, from a newly placed catheter in subjects with long-term indwelling urethral catheters, or by aspiration from the catheter port of a short-term indwelling catheter. Quantitative criteria for the microbiologic diagnosis of urinary infection are shown in Table 108.3. Lower quantitative counts in subjects with indwelling catheters often reflect contamination from catheter biofilm rather than bladder bacteriuria.

A diagnosis of symptomatic urinary infection requires a positive urine culture. However, a positive urine culture is common in catheterized patients at any time. Patients with short-term indwelling catheters have an increasing prevalence of bacteriuria the longer the catheter remains in situ; those maintained on intermittent catheterization have a prevalence of about 50% at

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