Clinical features of Escherichia coli positive calcium oxalate stones and effects of outer membrane vesicles on tubular epithelial injury and crystal adhesion.

BACKGROUND: Although calcium oxalate (CaOx) stones are classically regarded as a metabolic stone type, bacterial factors may also contribute to clinical heterogeneity. However, the biological relevance of bacteriuria in CaOx stone disease remains unclear. We investigated the clinical association between bacteriuria and CaOx stone phenotype and examined whether Escherichia coli-derived outer membrane vesicles (OMVs) could induce epithelial changes relevant to crystal retention. METHODS: We retrospectively analyzed 1,976 patients with urinary stones treated at a single center between 2016 and 2020, including 1,277 with CaOx stones, 465 with infection stones, and 234 with uric acid stones. Patients with CaOx stones were further stratified by urine culture status and bacterial species. In parallel, OMVs were isolated from a uropathogenic E. coli reference strain and characterized by transmission electron microscopy and nanoparticle tracking analysis. Their effects on HK-2 cells were assessed using confocal internalization assays, CCK-8, reactive oxygen species detection, oxidative stress-related biochemical assays, and calcium oxalate monohydrate (COM) crystal adhesion assays. RESULTS: CaOx, infection, and uric acid stones exhibited distinct clinical characteristics and different measured urinary biochemical parameters. Within the CaOx cohort, urine culture-positive patients had larger stones and a higher proportion of renal stones than culture-negative patients despite lower 24-hour urinary excretion of several ions. Among patients with culture positive CaOx stones, those positive for E. coli were characterized by older age, female predominance, renal stone predominance, larger maximum stone diameter, and lower urinary excretion of several ions measured over 24 hours, whereas the pooled other bacteria group showed a less uniform pattern. E. coli-derived OMVs were internalized by HK-2 cells in a time-dependent manner, reduced cell viability, increased reactive oxygen species and malondialdehyde, decreased superoxide dismutase and catalase activity, and enhanced COM crystal adhesion. CONCLUSION: Clinical and experimental findings suggest that E. coli-associated bacterial signals may be relevant to a subset of CaOx stone disease and provide biological plausibility for E. coli-derived OMVs as cell-free bacterial effectors that may link bacteriuria to tubular epithelial injury and crystal retention.