Mechanistic insights into phage dynamics to overcome intracellular bacterial reservoirs in urinary tract infections

Recurrent urinary tract infections frequently arise from uropathogenic bacteria invading bladder epithelial cells, where they establish persistent intracellular reservoirs. Phages represent a promising therapeutic alternative to antibiotics owing to their host specificity, ability to circumvent antibiotic resistance, and capacity to disrupt biofilms. In this study, we investigated whether lytic phages can gain access to human bladder epithelial cells and are able to target internalized bacteria using an in vitro bladder infection model. We show that phages can enter the bladder epithelium either via direct cellular uptake or through co-invasion with extracellularly infected bacteria. Notably, the presence of extracellular uropathogenic Escherichia coli (UPEC) substantially enhanced intracellular phage uptake, revealing a previously unrecognized bacteria-assisted pathway for intracellular phage delivery that may facilitate access to intracellular bacterial reservoirs. Phage treatment prevented epithelial infection by UPEC and preserved urothelial cell integrity. Moreover, phages reduced intracellular bacterial burden, although complete eradication was not achieved and efficacy was limited to early stages of infection. Together, these findings advance understanding of phage-pathogen-human interactions to optimize phage-based strategies for recurrent urinary tract infections.