Intravesical mycobacteria reshape bladder and gut microbiota in a murine bladder cancer model

The bladder microbiota may influence tumor progression and responses to intravesical therapies, yet the effects of Mycobacterium bovis BCG instillations on bladder and gut microbiota remain unexplored. This study evaluates, for the first time, the impact of M. bovis BCG and Mycobacterium brumae, a non-pathogenic immunomodulator, on microbiota composition in bladder tissue and cecal stool of healthy and tumor-bearing mice. Mycobacterial instillations increased gut microbial richness but reduced Actinobacteria. In tumor-bearing mice, Muribaculaceae and Bacteroidaceae were enriched, while M. bovis BCG reduced Muribaculaceae and increased Rikenellaceae. In the bladder, M. brumae instillations favored Staphylococcaceae in healthy mice, while Enterococcaceae dominated most groups except M. bovis BCG-treated tumor-bearing mice. Escherichia/Shigella decreased in mycobacteria-treated tumor-bearing mice, while Burkholderia was more prevalent in untreated bladders. Notably, M. brumae increased stool alpha diversity, whereas M. bovis BCG reduced bladder richness. These findings reveal that intravesical mycobacterial therapy modulates microbiota composition across body sites, underscoring the complex host-tumor-immunotherapy interplay.