P002

1:Universitat Autònoma de Barcelona; 2:Institute of Agrochemistry and Food Technology-National Research Council (IATA-CSIC), Valencia; 3:Instituto de Biomedicina de Valencia (IBV), CSIC; 4:Institute for Bioengineering of Catalonia (IBEC); 5:Universitat de Barcelona

The once-held belief that the urinary tract was sterile has been challenged by the discovery of its microbiota. Some studies have observed differences in microbiota between healthy individuals and bladder cancer patients, although results show disparity, and a clear differentiated profile has not been proven. Bladder microbiota may affect tumour progression and response to therapies, including Mycobacterium bovis BCG instillations. The interference or synergy mechanisms between bladder microbiota and BCG treatment remain unknown. This study evaluates for the first time the impact of intravesical instillations of BCG and Mycobacterium brumae, a non-pathogenic immunomodulatory species, on microbiota composition of bladder tissue and cecal stool in healthy and tumour-bearing mice.

Our results showed that mycobacterial instillations enhanced microbial richness but reduced Actinobacteria abundance across groups in the gut. At the family level, Muribaculaceae and Bacteroidaceae were significantly enriched in tumour-bearing mice, whereas BCG-treated animals showed lower Muribaculaceae and higher Rikenellaceae abundance. Regarding bladder microbiota, Staphylococcaceae dominated in M. brumae-treated healthy mice, while Enterococcaceae was abundant in all other groups except BCG-treated tumour-bearing mice. At genus level, Escherichia and Shigella were notably reduced in mycobacteria-treated tumour-bearing mice, while Burkholderia was more prevalent in untreated bladders. Alpha diversity increased in cecal stool following M. brumae instillations, regardless of tumour status. In contrast, BCG reduced bladder microbiota richness in tumour-bearing mice. Beta diversity analyses revealed subtle, non-significant clustering among groups.

These findings highlight how intravesical mycobacterial therapy modulates microbiota composition in distinct anatomical sites, underlining the complex interplay between host, tumour, and immunotherapy.