Previous experimental investigations of the crystalline biofilms that colonize and block urinary catheters have focussed on their formation by pure cultures of Proteus mirabilis. In the urine of patients undergoing long-term catheterization, P. mirabilis is commonly found in mixed communities with other urinary tract pathogens. Little is known about the effect that the other species have on the rate at which P. mirabilis encrusts catheters. In the present study, a set of data on the nature of the bacterial communities on 106 catheter biofilms has been analysed and it was found that while species such as Providencia stuartii and Klebsiella pneumoniae were commonly associated with P. mirabilis, when Escherichia coli, Morganella morganii or Enterobacter cloacae were present, P. mirabilis was rarely or never found. The hypothesis that the absence of P. mirabilis from some biofilm communities could be due to its active exclusion by other species has also been examined. Experiments in laboratory models showed that co-infection of P. mirabilis with M. morganii, K. pneumoniae or E. coli had no effect on the ability of P. mirabilis to encrust and block catheters. Co-infection with Ent. cloacae or Pseudomonas aeruginosa, however, significantly increased the time that catheters took to block (P ,0.05). The growth of Ent. cloacae, M. morganii, K. pneumoniae or E. coli in the model for 72 h prior to superinfection with P. mirabilis significantly delayed catheter blockage. In the case of Ent. cloacae, for example, the mean time to blockage was extended from 28.7 h to 60.7 h (P ¡0.01). In all cases, however, P. mirabilis was able to generate alkaline urine, colonize the biofilms, induce crystal formation and block the catheters. The results suggest that although there is a degree of antagonism between P. mirabilis and some of the other urinary tract organisms, the effects are temporary and whatever the pre-existing urinary microbiota, infection with P. mirabilis is thus likely to lead to catheter encrustation and blockage.

Aims:  Infection of the catheterized urinary tract with Proteus mirabilis causes blockage of the catheter by crystalline bacterial biofilms. The aim of this work is to identify a surface‐coating for catheters that is not vulnerable to colonization by Pr. mirabilis. Methods and Results:  A parallel‐plate flow‐cell and phase contrast microscopy were used to follow bacterial adhesion onto polymer films. Experiments with a urease‐negative mutant of Pr. mirabilis suspended in buffer or urine, identified agarose as a polymer on which biofilm did not form. In tests with wild‐type urease‐producing cells in urine, no adhesion of cells onto agarose was observed for 3 h but then as the pH rose above 8·2, the surface rapidly became colonized by crystalline biofilm. Conclusions:  In urine at pH below 8·0, Pr. mirabilis does not adhere to agarose‐coated surfaces. When the pH rises above 8·2, however, aggregates of crystals and bacteria form in the urine and are deposited on such surfaces. Significance and Impact of the Study:  Strategies to prevent the formation of crystalline biofilms on urinary catheters will need to consider both the properties of the surface‐coatings and the requirement to prevent the alkaline conditions that induce crystal formation in urine.