OBJECTIVE: produces a genotoxin, cytolethal distending toxin (CDT), which has DNAse activity and causes DNA double-strand breaks. Although infection has been shown to promote intestinal inflammation, the impact of this bacterium on carcinogenesis has never been examined. DESIGN: Germ-free (GF) mice, fed with 1% dextran sulfate sodium, were used to test tumorigenesis potential of CDT-producing . Cells and enteroids were exposed to bacterial lysates to determine DNA damage capacity via γH2AX immunofluorescence, comet assay and cell cycle assay. To examine the interplay of CDT-producing , gut microbiome and host in tumorigenesis, colonic RNA-sequencing and faecal 16S rDNA sequencing were performed. Rapamycin was administrated to investigate the prevention of CDT-producing -induced tumorigenesis. RESULTS: GF mice colonised with human clinical isolate 81-176 developed significantly more and larger tumours when compared with uninfected mice. with a mutated subunit, mut, attenuated -induced tumorigenesis in vivo and decreased DNA damage response in cells and enteroids. infection induced expression of hundreds of colonic genes, with 22 genes dependent on the presence of c The -infected group had a significantly different microbial gene expression profile compared with the mut group as shown by metatranscriptomic data, and different microbial communities as measured by 16S rDNA sequencing. Finally, rapamycin could diminish the tumorigenic capability of . CONCLUSION: Human clinical isolate 81-176 promotes colorectal cancer and induces changes in microbial composition and transcriptomic responses, a process dependent on CDT production.