Catheter Associated Urinary Tract Infection
Indwelling bladder catheters constitute a convenient way to relieve urinary retention and manage long-term urinary incontinence. Unfortunately they also provide easy access for bacteria from a contaminated external environment into a vulnerable body cavity.
Organisms colonizing the periurethral epithelia and skin can migrate to the bladder over the external surface of the catheter. The taps of the urine drainage bags can also become contaminated during use and their regular opening to drain the urine allows bacteria to pass into the bags. Urine is an excellent growth medium for bacteria and the subsequent multiplication of the contaminating organisms produces dense bacterial populations in the bags. These become reservoirs of infection from which bacteria can migrate through the drainage tube and catheter to the bladder. Disconnection of the catheter from the drainage tube can also produce contamination of the system.
Development of infection
The risk of urinary tract infection is related to the length of time the catheter is in place. Patients undergoing long-term catheterisation (>28 days) will inevitably acquire infections, even with careful management.
In the early stages, infection is usually by single species of bacteria such as Staphylococcus epidermidis, Enterococcus faecalis or Escherichia coli. The longer the catheter remains in place however, the greater the variety of organisms that accumulate in the residual urine in the bladder. Long-term catheter users commonly become infected with complex mixed communities of bacterial species. Organisms such as Providencia stuartii, Pseudomonas aeruginosa, Klebsiella pneumoniae and Proteus mirabilis are particularly common and persistent. These polymicrobial populations are difficult to eliminate with antibiotic therapy while the catheter remains in place. The infections are generally asymptomatic and it is common practice not to treat with antibiotics until clinical symptoms suggest the kidneys or the bloodstream have become involved.
Long-term catheter users commonly have their catheters changed at 8 to 12 week intervals. This means that infected urine can be flowing through the catheters continuously for periods of up to three months.
Bacterial biofilms
Bacteria have a marked preference for life attached to surfaces rather than in aqueous suspension. Attached to surfaces and bathed in a flow of nutritious growth medium, they multiply rapidly and secrete a polysaccharide gel around themselves as a protection against environmental stresses. These communities are known as bacterial biofilms, and this biofilm mode of growth seems to be a basic survival strategy for bacteria.
Many types of implanted medical devices in contact with body fluids are susceptible to biofilm colonization and it is clear that cells in these biofilms are resistant to phagocytes, antibodies and antibiotics. These properties are responsible for the characteristic failure of the host defence systems and antimicrobial chemotherapy to clear device-associated infections.
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