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Catheter-associated urinary tract infections (CAUTI) are complex infections often involving multi-species bacteria. Escherichia coli is frequently an early coloniser. Subsequent colonisation by Pseudomonas aeruginosa and coexistence mechanisms between the two strains within urethral catheters is not yet fully understood. In this study, metabolic adaptations between co-isolated clinical E. coli and P. aeruginosa strains were investigated. It was found that P. aeruginosa outgrew E. coli in artificial urine medium (AUM), whereas E. coli dominated in culture broth such as Iso-sensitest. No evidence of direct antagonism was observed. Metabolite analyses revealed distinct metabolite patterns indicating cross-feeding and metabolic adaptations. In AUM, stress-response metabolites were elevated. Additionally, E. coli appeared to experience Fe-limitation in AUM, while the same was not observed for P. aeruginosa. The results highlight the influence of nutrient conditions on processes within mixed biofilms.

Introduction: Bacterial colonization of medical devices is promoting hospital-acquired infections leading to worsening patient outcomes and high costs for society. Sequential bacterial colonization of surfaces may provide altered conditions that benefit pathogens.

Methods: In this study we have investigated the interactions between two pairs of clinical isolates collected from patients that were on mechanical ventilation. Two patients were first colonized by Staphylococcus aureus and thereafter Pseudomonas aeruginosa settled. The two P. aeruginosa isolates were weak colonizers in monoculture. We investigated two hypotheses: (1) S. aureus preconditions material surfaces, facilitating adhesion of later colonizers. (2) S. aureus provides an altered nutrient environment promoting the growth and settlement of other bacteria.

Results: Surface preconditioning did not seem to enhance colonization of P. aeruginosa. However, bacterial growth, biofilm formation, ratio of colony forming units, and metabolic profiles were influenced by co-cultivation. The effects varied depending on nutrient content in the medium.

Discussion: In general, co-cultures appeared to benefit clinical isolates to a higher degree, compared to reference strains. The results indicate that differences in airway microenvironment between patients may have a large effect on the infection process and which pathogens that persist.

There is a great interest in developing novel anti-biofilm materials in order to decrease medical device-associated bacterial infections causing morbidity and high healthcare costs. However, the testing of novel materials is often done using bacterial lab strains that may not exhibit the same phenotype as clinically relevant strains infecting medical devices. Furthermore, no consensus of strain selection exists in the field, making results very difficult to compare between studies. In this work, 19 clinical isolates of Pseudomonas aeruginosa originating from intubated patients in an intensive care unit have been characterized and compared to the lab reference strain PAO1 and a rmlC lipopolysaccharide mutant of PAO1. The adhesion and biofilm formation was monitored, as well as cell properties such as hydrophobicity, zeta potential and motility. Two groups of isolates were observed: one with high adhesion to polymer surfaces and one with low adhesion (the latter including PAO1). Furthermore, detailed biofilm assays in a flow system were performed using five characteristic isolates from the two groups. Confocal microscopy showed that the adhesion and biofilm formation of four of these five strains could be reduced dramatically on zwitterionic surface coatings. However, one isolate with pronounced swarming colonized and formed biofilm also on the antifouling surface. We demonstrate that the biofilm properties of clinical isolates can differ greatly from that of a standard lab strain and propose two clinical model strains for testing of materials designed for prevention of biofilm formation in the respiratory tract. The methodology used could beneficially be applied for screening of other collections of pathogens to identify suitable model strains for in vitro biofilm testing.

Statement of Significance: Medical-device associated infections present a great challenge in health care. Therefore, much research is undertaken to prevent bacterial colonization of new types of biomaterials. The work described here characterizes, tests and presents a number of clinically relevant bacterial model strains for assessing biofilm formation by Pseudomonas aeruginosa. Such model strains are of importance as they may provide better predictability of lab testing protocols with respect to how well materials would perform in an infection situation in a patient. Furthermore, this study uses the strains to test the performance of polymer surfaces designed to repel bacterial adhesion and it is shown that the biofilm formation for four out of the five tested bacterial strains was reduced.