Preventing Catheter-Associated Urinary Tract Infections

Laura A. Stokowski, RN, MS

Disclosures

February 03, 2009

Urinary Catheter Colonization and Infection

Colonization of urinary catheters is inevitable and expected. Once microorganisms colonize the urine, they rapidly progress, within 72 hours, to concentrations > 105 colony-forming units (CFU)/mL.[4] The longer the catheter is in place, the more likely colonization will occur. It is believed that extraluminal colonization is the most likely route of entry for microorganisms, particularly via the shorter female urethra.[5]

Biofilm

Colonization of a urinary catheter is perpetuated by microorganism-produced biofilm. Following insertion of a standard urinary catheter, a conditioning film made up of proteins, electrolytes, and other components of urine is deposited on the surface of the catheter.[6] Microbes attach to this conditioning film and begin secreting polysaccharides that form the architectural structure of biofilm (Figure).[7] Biofilms generated by gram-negative organisms, gram-positive organisms, or yeasts are a survival strategy for these microorganisms, offering protection from both the body's defenses and antimicrobial agents.[8]

Initially composed of a single species, the biofilm on a long-term indwelling urinary catheter can also contain multiple species,[7] with mixed-organism biofilms containing as many as 16 different strains of bacteria.[9] Biofilms are both tenacious and resistant to antimicrobial agents, explaining their importance in the pathogenesis of catheter-associated infection.[7] Under unfavorable conditions, organisms can detach from the biofilm and become free-floating. Free-floating organisms in the urine can lead to symptomatic infection.[6]

Figure

Electron micrograph depicting round Staphylococcus aureus bacteria, with biofilm, the sticky-looking substance woven between the bacteria. (Content source: Rodney M. Donlan, PhD; Janice Carr, Public Health Image Library, Centers for Disease Control and Prevention; 2005.)

In a urinary catheter, the biofilm produces urease, which hydrolyzes urea in the patient's urine to ammonium hydroxide. The elevated pH can precipitate minerals in the biofilm that then encrust and block the lumen of the catheter.[7] Proteus mirabilis, a urease-producing organism, is often responsible for encrusting a catheter with calcium phosphate as quickly as 18 hours after insertion. Encrustation-resistant catheters must contain antimicrobial agents that diffuse into the urine and prevent the rise in pH that triggers crystal formation. Otherwise, the bacteria can simply attach to the crystalline foundation layer and continue to grow. The bottom line is that acidifying the urine without removing the source of urease will not prevent encrustation.

Mechanisms of Colonization

Most microorganisms causing CAUTI derive from the patient's own colonic and perineal flora (such as Escherichia coli) or from the hands of healthcare personnel during catheter insertion or manipulation of the collection system.[5] Extraluminal contamination can occur by direct inoculation of the catheter tip from organisms on the external urethral meatus,[5] and subsequent dragging of organisms along the full length of the urethra as the catheter is inserted fully into the bladder.[9] The tip is also the likely source of bacteria found on the inside of catheters that are carried downward by the flow of urine.[9] It has also been hypothesized that perineal microorganisms, such as enterococci, staphylococci, and Candida species, ascend in the space between catheter and urethra,[10] traveling by capillary action in the thin mucous film surrounding the catheter. Studies indicate that there are more bacteria outside the catheter than inside.[9] Bacteria from the outer surface of the catheter then colonize the bladder epithelium, contaminating the urine.[9]

The usually sterile urinary tract can also become colonized in an ascending manner from the drainage bag or the catheter-bag junction, up through the lumen of the catheter, although these routes are considered less likely than colonization from the flora of distal urethra. Transport of a large quantity of intraluminal contaminants into the bladder can theoretically occur by retrograde reflux of contaminated urine when the catheter or collection system is moved or manipulated.[5] The clinical significance of these potential routes of infection is unknown.

Risk Factors for CAUTI

Duration of catheter use is the most important risk factor for the development of infection.[1] The risk of CAUTI increases by an estimated 5% to 10% each day that the catheter remains in place. Patients with long-term catheters are almost assured of developing CAUTI. Urinary stasis and overdistention are additional risk factors for bacteriuria and potential infection. Other risk factors are female sex , older age, and failure to maintain a closed drainage system.[1] The urine collection bag is a reservoir for microorganisms, and handling it properly can reduce the risk for infection. Susceptibility of the host (neonates, elderly patients, immunocompromised patients) is an additional risk factor for infection.

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