Urinary Microbiome Characteristics in Kidney Transplant Recipients and Their Clinical Implications: A Narrative Review

High-throughput sequencing has overturned the long-standing "sterile urine" paradigm and revealed a low-biomass yet clinically informative urinary tract microbiota. In kidney transplant recipients, immunosuppression, perioperative instrumentation, and antibiotic exposure can reshape urinary microbial communities; however, reported signatures remain heterogeneous across cohorts and methodologies. This narrative review synthesizes evidence on: (1) baseline urobiome patterns and major determinants of inter-individual variability, (2) post-transplant drivers of dysbiosis, and (3) associations between urobiome dynamics and key transplant outcomes, including urinary tract infection (UTI), acute rejection (AR), and chronic allograft dysfunction such as interstitial fibrosis and tubular atrophy (IF/TA). Across studies, dysbiosis commonly manifests as reduced diversity, depletion of putatively protective taxa, and enrichment of opportunistic pathogens; several longitudinal cohort studies further suggest that microbiome shifts can precede clinical events, supporting a potential window for risk stratification and early surveillance. We also summarize translational research directions, including integration of urinary microbial profiles with host biomarkers and multi-omics readouts, as well as microbiome-sparing strategies (antimicrobial stewardship, targeted probiotics/synbiotics, and dietary modulation). Finally, we highlight methodological challenges unique to low-biomass urine samples - especially contamination control, negative controls, and transparent reporting - that are essential for improving reproducibility and enabling clinical implementation. This review aims to provide an up-to-date, clinically oriented synthesis of the post-transplant urobiome and to propose methodological and translational priorities for future research and implementation.