The Urinary Microbiota and the Gut–Bladder Axis in Bladder Cancer

The human bladder hosts a resident, low-biomass microbial community (urobiota) that has only become the subject of intense investigation in the last 15 years. The advantages that the urobiota may confer to the bladder, in contrast to the microbiota of other mucosal sites, remain to be elucidated. Alterations in the urobiota have been associated with various pathological urogenital conditions, including urinary tract infections (UTIs) and recurrent UTIs. A potential link between bladder cancer (BC), the ninth most common human cancer by incidence worldwide, and a dysbiotic urobiota is still unclear and represents an emerging field of study. In this review, we focus on recent studies that not only analyzed the urobiota of BC patients using urine specimens to identify biomarkers and microbial signatures of the disease, but also monitored therapeutic responses to therapies. We also discuss novel techniques of culturing, such as culturomics, animal models of BC, and 3D organotypic models. Furthermore, we review studies on the gut–bladder axis which, though still limited, already suggest that diet- and gut-derived bacterial metabolites can influence BC progression and individual responses to therapy.