Hypothesis / aims of study
Urinary tract infections (UTI) are a significant concern for users of intermittent catheters (ICs). The aetiology of infections is primarily from two sources: users hands during handling of the catheter and transfer of bacteria from the distal urethra to the bladder by the catheter itself (1).
Several manufacturers produce ICs with UTI prevention technology, such as insertion tips which aim to prevent UTIs by reducing catheter contamination from the distal urethra (which is colonised with microorganisms) thereby reducing the volume of bacteria entering the bladder during catheter insertion.
However, few in vitro models are reported in the literature that can aid in the understanding of intermittent catheterisation associated UTIs and test such technologies. The bacterial displacement method (adapted from a previously used method (2)) was therefore developed to simulate the transfer of bacteria during insertion of an IC along a simulated urethral agar channel (UAC). We used this in vitro microbiological testing method to assess several commonly used closed system ICs and their performance in terms of reducing bacterial displacement.
Study design, materials and methods
ICs evaluated in the model include: Cure closed system (CCS) with insertion tip; Vapro Plus Pocket Set (VPP) with full length handling sleeve and insertion tip; SpeediCath Flex Set (SCF) with full length handling sleeve only; SpeediCath Compact Set (SCC) with no sleeve or insertion tip. All ICs also included a urine bag attached.
Testing was performed in vitro using n=3 replicates for each catheter for both bacterial counts and visual displacement images, using the challenge organisms Escherichia coli NCIMB 14067 and Enterococcus faecalis NCTC 12201; two commonly found bacterial causes of UTIs in intermittent catheter use (3).
UAC preparation: Selective agar channels were made by aseptically dispensing 30ml of molten agar into 30ml universal containers. Sterile 4mm stainless steel rods were placed into the universal containers and removed once the agar was set to create a channel down the centre the same diameter as CH12 catheters.
UAC inoculation: A sterile swab was placed into the 50µl inoculum of the 1x10^7 CFU/ml bacterial suspension. The inoculated swab was then inserted 1cm into the insertion entrance of the UAC, rotated anticlockwise twice and removed. This simulates the distal urethral bacterial contamination area. This was performed for all the test and bacterial growth control UACs. Enough UACs were prepared for the above samples so that both bacterial total viable counts (TVC) and images of the displacement of bacteria could be reviewed.
Catheter insertion: catheters were hydrated and handled as per manufacturer instructions, then slowly inserted into the UAC, and left in place for 2 minutes to simulate catheter use to drain urine.
Microbiological Analysis: Following insertion and removal of the catheter, UACs were incubated at 35±3°C for 24 hours, then tipped from the container and split in half lengthways to allow imaging of the displacement of bacteria through the UAC. Bacterial TVC were also performed by cutting UACs into 7x 1cm sections starting at the insertion entry of the UAC. The 1st (where inoculation has occurred and represents the meatus (distal urethra)), 3rd, 5th and 7th centimetre sections were microbiologically blended so that the bacteria were released from the agar and appropriate dilutions plated onto tryptone soy agar (TSA) plates. Plates were incubated at 35±3°C for 48 hours and numbers of bacteria in each section counted following incubation. In addition, light microscopy and confocal laser scanning microscopy was performed during testing to evaluate the whereabouts of the bacteria on the catheters following insertion.
Results
Bacterial displacement results for the growth control UAC, where no catheter had been inserted, showed high numbers (approximately 3 x 10^5 cfu/section) in the 1st section of the UAC as expected, and no bacteria by the 3rd section confirming that, if no catheter was used, the bacteria had not moved along independently. Bacterial displacement along the UAC was shown following insertion of VPP, SCF and SCC catheters via the images taken and the bacterial TVC.
Following insertion of catheters, E. coli TVC results obtained showed the 1st section in the UAC (where bacterial inoculation occurs), had the most bacteria at 3 x 10^5 cfu/section for all the catheters tested. Catheters VPP, SCF and SCC had bacteria present in the 7th section at numbers of 3 x 10^2, 3 x 10^2 and 6 x 10^1 cfu/section, respectively. In contrast, the CCS catheter exhibited reduced bacterial displacement, with bacterial numbers at undetectable levels (<30CFU) by the 3rd section, which is the same as obtained by the growth control UAC.
A similar pattern of bacterial displacement was also observed for E. faecalis testing, where VPP, SCF and SCC demonstrated bacterial displacement along the length of the UAC whereas CCS showed undetectable levels by the 3rd section of the UAC. Interestingly numbers displaced along the UAC for all samples except CCS were higher than that obtained for E. coli, suggesting that E. faecalis has a higher affinity to the catheter material.
Interpretation of results
A range of catheters were compared; some with tips, and some without. All ICs except the CCS catheter demonstrated bacterial displacement along the full length of the simulated urethral agar channel (UAC).
It was expected that ICs with insertion tips would have improved performance in the testing, as insertion tips are understood to bypass the distal urethra therefore minimising contact between bacteria and the IC; however, this was not consistently found. In this test, results varied, and the CCS catheter insertion tip was found to perform better than the VVP catheter insertion tip in preventing bacteria transfer along the UAC. On visual inspection of the insertion tips, gaps were observed between the cross-cut opening of the VVP insertion tip. In contrast, the CCS insertion tip enclosed the IC more thoroughly and hence we propose that this design provides an effective bacterial barrier.