Prior studies have found that several Lactobacillus species inhibit the growth of or kill uropathogenic Escherichia coli (UPEC). Lactobacillus crispatus, L. gasseri, L. iners, and L. jensenii are frequent dominant members of both the healthy female vaginal and urinary microbiota and have been associated with the lack of lower urinary tract symptoms, including urinary tract infections (UTIs). Urinary Lactobacilli produce hydrogen peroxide (H2O2) and lactic acid, both of which can stress the UPEC cell and are attributed to reduced colonization within the bladder. As phage biologists and the first group to investigate phages within the urobiome, we recognized an immediate connection between Lactobacillus-dominated environments and temperate phage induction. Our prior genomic survey of bacteria in the urinary tract (urobiome) found that most bacterial genomes have latent (integrated) phage genomes, and our prior empirical work suggests that most of these latent phages are in fact temperate phages capable of being induced. The induced phage excises from the bacterial genome, replicates, and lyses (kills) its bacterial host cell. Our proof-of-concept work found: (i) cell-free Lactobacillus supernatant can induce UPEC latent temperate phages (coliphages), (ii) liquid cultures at Lactobacillus-relevant pH values can induce UPEC temperate coliphages, and (iii) liquid cultures at Lactobacillus-relevant H2O2 levels can induce UPEC temperate coliphages. This leads to our hypothesis: Lactobacilli may control UPEC colonization via “lysis from within” – H2O2 and/or pH stress UPEC strains, which leads to induction of resident temperate coliphages and death of the UPEC cell. Taking an interdisciplinary approach, the proposed project will assess the frequency and cause of induction of UPEC coliphages by Lactobacillus. Aim 1 will expand our preliminary work to determine if the cell-free supernatant of all common urinary Lactobacillus species induces UPEC coliphages; multiple strains of L. crispatus, L. gasseri, L. iners, and L. jensenii will be tested on a large collection of E. coli isolated from the bladders of females with and without LUTS, including UTI. In Aim 2, we will ascertain if phage induction triggered by cell-free Lactobacillus supernatant is the result of stress triggered by pH and/or H2O2 levels. Additionally, we will determine how pH and H2O2 impact UPEC growth over time. Our investigation will determine what role phage induction plays in shaping the urobiome community. Understanding the mechanisms that control UPEC colonization can inform clinical care of acute UTIs. This project is proposed as an R15 AREA for Undergraduate-Focused Institutions and is designed to engage undergraduates in interdisciplinary research of benign urologic disease.