Microbial biofilms on peritoneal dialysis catheters
Abstract: When the kidneys are failing the blood needs to be purified artificially. This can be done using peritoneal dialysis, in which a catheter is surgically inserted into the peritoneal cavity, through the abdominal wall, and used to infuse or withdraw dialysis fluid. When handling the catheter, touch contamination may occur and this can result in bacterial access to the peritoneal cavity, formation of biofilms and subsequent infections. Peritonitis, an often very painful inflammation of the peritoneum, is the major cause of morbidity in peritoneal dialysis patients and often leads to hospitalisation during treatment. Thus, the benefits of reducing the number on infections would be huge, with better quality of life for the patients and a reduction in costs for society. The aim of this thesis was to study bacterial colonisation on catheters from peritoneal dialysis patients as well as to study biofilm formation and microbial interactions on catheters in vitro. In a clinical study we investigated peritoneal dialysis catheters removed from patients due to renal transplantation (15 catheters) or infections (2 catheters). Bacterial colonisation was detected on 14 of the 17 catheters, as seen using confocal laser scanning microscopy or microbiological cultures, although the bacterial load was rather low. The most common species found on the catheters were Staphylococcus epidermidis and Propionibacterium acnes, both of which are low-virulent, commensal skin bacteria. Although usually low-virulent, S. epidermidis is the most common cause of medicaldevice related infections, and can cause persistent infections that are hard to eradicate. Due to its common presence in peritoneal dialysis catheter-related infections, S. epidermidis was further studied, together with another common opportunistic pathogen, Pseudomonas aeruginosa, often associated with more severe peritonitis cases which require catheter removal. These bacteria were cultured together and, interestingly, we found the number of S. epidermidis cells constantly declined compared to the P. aeruginosa cells, even when they were originally present in equal amounts, suggesting that P. aeruginosa dispersed the S. epidermidis biofilms. This dispersal effect of P. aeruginosa on S. epidermidis biofilms was seen for clinical and laboratory strains of both species. When S. epidermidis biofilms were exposed to cell-free growth medium of P. aeruginosa biofilms, the same effect could be seen, demonstrating that it was due to extracellular substance(s). Several extracellular substances of P. aeruginosa were investigated for their dispersal effect, including proteases, a quorum sensing-related homoserine lactone, the toxin pyocyanin and polysaccharides. For various reasons, all were excluded as the potentially active substance except the polysaccharides. This result was further supported when the cell-free growth medium of P. aeruginosa was fractionated and studied for S. epidermidis dispersal effect. The material with the greatest effect on S. epidermidis was a fraction rich in polysaccharides as shown by PAS staining. This pool contained no detectable mannose or galactose residues, and may correspond to the glucose-rich polysaccharide product of the pel gene cluster. Other fractions, including a galactose- and mannose-rich polysaccharide possibly corresponding to the psl gene product as well as one that was protein-rich were also active but to a lesser extent. Other bacterial polysaccharides have exhibited antibacterial effects, probably due to altered cell integrity, and possibly the potential P. aeruginosa polysaccharide might function in a similar way. In addition to functioning as a dispersal agent of established biofilms, the P. aeruginosa supernatant also seemed to inhibit biofilm formation by preventing adsorption of proteins to the catheter surface. In the future, identification of the substances responsible for these effects would allow them to be used in new strategies to prevent formation or eradicate biofilms from peritoneal dialysis catheters, thus reducing the number of infections.
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