Influence of microbial composition and sample type on antimicrobial resistance in urinary tract infections: a single-centre retrospective cohort study (2015-2023)

ObjectivesTo quantify how urine sample type and polymicrobial context impact antimicrobial resistance (AMR) in urinary tract infections (UTIs), using routine diagnostics at scale. MethodsIn this retrospective, single-centre study, we analysed 188,687 urine cultures from the Institute of Medical Microbiology, University of Zurich, Switzerland (January 2015 to May 2023). We compared midstream urine (MU), indwelling catheter (IDC), and intermittent catheter (IMC) samples. Samples were classified as negative, bacteriuria, or UTI, by meeting a microbiological UTI threshold ([&ge;]105 CFU/mL). We compared sample types using covariate-adjusted regression and constrained ordination for community composition. In bimicrobial cultures, we assessed co-occurrence using adjusted pairwise odds ratios and degree-preserving permutation null models, supported by partner-choice analyses. AMR was modelled as acquired resistance (AR) and total resistance (TR: acquired + intrinsic) probabilities, with predictor contributions quantified using mutual information. ResultsAmong 186,819 MU, IMC, IDC samples, 56,867 met the UTI threshold. Catheter-associated UTIs (IDC and IMC) were ~60% more likely to be polymicrobial than MU samples. Community composition differed by sample type (p<0{middle dot}001). In IDC, Escherichia coli was less prevalent than in MU, but device-associated pathogens like Pseudomonas aeruginosa and Candida albicans were enriched. Most species-pairs showed no increased co-occurrence after adjusting for covariates, but a subset showed reproducible enrichment across methods (e.g., C. albicans-C. glabrata). Organism identity was the dominant determinant of AMR, with the highest mutual information across AR and TR. AR was higher in IDC for common uropathogens (e.g., E. coli). Co-isolation with hospital-associated partners (e.g., Enterococcus faecium) was associated with further AR increase. From 2015 to 2023, AR increased from ~48% to ~60%, with rising {beta}-lactam (+{beta}-lactamase inhibitor) resistance and declining fluoroquinolone resistance in Enterobacterales. ConclusionsSample type and co-isolated partners provide clinically actionable information beyond pathogen identity and could support more context-aware reporting and empiric prescribing.