Novel bacterium sp. M3 promotes colorectal tumorigenesis via the production of the carcinogen styrene.

[BACKGROUND] The bacterial marker 'm3' shows promise for the non-invasive diagnosis of colorectal cancer (CRC) and adenomas. However, the m3-harboring bacterium (M3) had not been successfully cultured.

[OBJECTIVE] This study aims to elucidate the functional mechanisms of M3 in CRC.

[DESIGN] M3 was isolated using a targeted enrichment strategy. Its functional roles were investigated and . Bacterial and fecal metabolites were analyzed by untargeted LC-MS and targeted LC/GC-MS. RNA-seq profiled host gene expression altered by M3. Key enzymes were identified through whole-genome sequencing and proteomics.

[RESULTS] M3 is phylogenetically related to but exhibits distinct genetic and phenotypic characteristics. M3 significantly promoted colon tumor development in both mice and azoxymethane-treated mice. M3 culture supernatant enhanced colon cancer cell proliferation, migration, and cell cycle progression, accelerated xenograft tumor growth, stimulated intestinal organoid expansion, and disrupted DNA damage repair pathways. M3 produced styrene-a recognized human carcinogen-in both cultures and mouse models, a function not previously reported in bacteria. Importantly, styrene levels were significantly elevated in feces of CRC patients and exceeded WHO safety limit in mouse gut (12.5 vs. 7.7 μg/kg/d). Moreover, we identified novel bacterial enzymes-aspartate ammonia-lyase and uroporphyrinogen decarboxylase-that convert phenylalanine to styrene.

[CONCLUSIONS] This study identifies M3 as a novel pro-tumorigenic bacterium in CRC, capable of direct biosynthesis of the carcinogenic metabolite styrene. We provide the first evidence of bacterial styrene biosynthesis, unveiling a previously unrecognized mechanism by which gut bacteria may promote colorectal tumorigenesis.The bacterial marker m3 has been associated with colorectal cancer, but the bacterium carrying it had not been cultured.Styrene is a recognized environmental carcinogen linked to cancers, and its bacterial biosynthesis had not been reported.We isolated and characterized the novel bacterium M3, which carries the m3 marker. It is phylogenetically related to but possesses distinct genetic and phenotypic features.M3 promotes colorectal tumorigenesis through the production of tumor-promoting metabolites, including styrene.Fecal styrene levels are significantly elevated in colorectal cancer patients.We elucidated the mechanism of direct styrene biosynthesis in M3, identifying two novel enzymes-aspartate ammonia-lyase and uroporphyrinogen decarboxylase-that catalyze this process.