A Microbiologist Explains Catheter Associated UTIs

Content Producer Tina Samuels sits down with Peter Obee,a Microbiologist at the University of Wales at Cardiff, to explain catheter associated UTIs.

How common are catheter associated UTIs?
Catheter-associated UTI are the most common healthcare associated infections (HCAI), comprising approximately 40% of HCAI.

Why so high?
The presence of an indwelling catheter increases the likelihood of a patient acquiring a UTI, and the longer the catheter is in place the more the chances of acquiring a UTI increase.

This happens because the body’s defences are reduced by the presence of the catheter. Urine now trickles down the lumen of the catheter, and there is no longer a ‘flushing’ action through the urethra. The bladder will have a residual pool of urine in it, due to the design of the catheter. These factors mean that fewer bacteria are removed from the system than normally occurs.
In addition, the surface of the catheter is a much safer place for bacteria to exist than the surface of the urethra. Because of the ‘boundary layer’ that occurs very close to surfaces, it is much more difficult for the body’s natural defences to attack bacteria. Irritation caused by the urethra may make these defences less effective. However, the best route for bacteria is to reach the interior of the catheter. This often occurs if the drainage bag becomes contaminated, either when being attached, drained or even through general use. Another likely area for the introduction of bacteria is the joint between bag and catheter.

Once in the catheter, they are completely shielded from the body’s defences. It is an ideal spot for them being warm and bathed in a constant flow of nutrients. Bacteria that have reached the lumen of the catheter can attach to the surface and begin to grow. In time, they can make their way through the tube and into the bladder. Inside the bladder, the catheter’s design means that approximately 20ml of urine remains as a reservoir.

Treating UTIs in patients with catheter is difficult, because the catheter acts as a reservoir for the bacteria. Although most bacteria may be killed, because the bacteria in the catheter are protected from the antibiotics, they can re-establish an infection after the treatment is finished.

How does this occur?
As bacteria grow on the surface, they build up biofilm. This is a mixture of bacterial cells and a bacterial secretion. The slimy film in plug holes is an example of a bacterial biofilm. Cells in a biofilm are even more protected from external attack.
Some bacteria use urea to grow, which increases the pH of the urine. This causes soluble minerals present in the urine, such as calcium, to come out of solution and crystallize. These crystals can build up over time, causing the catheter to block. This can have serious consequences for the patient. This process also occurs on stents and can also be responsible for kidney stones.

Is there a prevention?
Some efforts have been made to prevent bacterial colonisation of catheters by impregnating the material with anti-microbials. Whilst these may have some short-term benefit, they are not long-term solutions. Dead bacteria and other organic matter will build up on the surface, providing a partial barrier to the anti-microbials and allow other bacteria to grow. Regular catheter replacement is therefore essential.

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