This study showed that the microbiota affects the healthy urinary bladder as well as the dynamics of the development of urinary bladder tumors. Axenic mice and those with antibiotic reduction of microbiota had decreased urinary bladder mass as well as changes in the expression of genes encoding extracellular matrix proteins and genes that control the circadian rhythm. The link between microbiota and the circadian rhythm has been described for other tissues, but this is the first evidence that it also exists in bladder tissue. Furthermore, the development of urinary bladder tumors is affected by the microbiota - antibiotic reduction of the microbiota greatly slows down the dynamics of tumor development caused by carcinogenic BBN. The most likely reason for the decrease in tumor dynamics is the reduced amount of the active carcinogen BCPN in the urine that was found in the BBN and antibiotics treated mice. It was shown ex vivo that the gut microbiota is capable of the production of the carcinogenic BCPN from BBN. Until now, it was known that the conversion of BBN to BPCN takes place in the liver, but this study showed that the intestinal microbiota also can perform this conversion and contribute to the total amount of this carcinogen. This is the first evidence of a causal relationship between intestinal microbiota and the development of bladder cancer, as well as the production of carcinogens in intestinal bacteria. The conducted research has resulted in fundamental biological discoveries that may be of clinical significance.