LAUSR

BACKGROUND & AIMS Dysbiosis of gut microbiota is linked to the development of colorectal cancer (CRC). However, microbiota-based stratification of CRC tissue and how this relates to clinico-molecular characteristics and prognosis remains to be clarified. METHODS Tumour and normal mucosa from 423 stage I-IV patients were profiled by bacterial 16S rRNA gene sequencing. Tumours were characterised for microsatellite instability (MSI), CpG island methylator phenotype (CIMP), APC, BRAF, KRAS, PIK3CA, FBXW7, SMAD4 and TP53 mutations; subsets for chromosome instability (CIN), mutation signatures and consensus molecular subtypes (CMS). Microbial clusters were validated in an independent cohort of 293 stage II/III tumours. RESULTS Tumours reproducibly stratified into three onco-microbial community subtypes (OCS) with distinguishing features: OCS1 (Fusobacterium/oral pathogens, proteolytic, 21%), right-sided, high-grade, MSI-high, CIMP-positive, CMS1, BRAF V600E and FBXW7 mutated; OCS2 (Firmicutes/Bacteroidetes, saccharolytic, 44%) and OCS3 (Escherichia/Pseudescherichia/Shigella, fatty acid beta-oxidation, 35%) both left-sided and exhibiting CIN. OCS1 was associated with MSI-related mutation signatures (SBS15, SBS20, ID2 and ID7), OCS2 and OCS3 with SBS18 related to damage by reactive oxygen species. Among stage II/III patients, both OCS1 and OCS3 had poorer overall survival as compared to OCS2 for microsatellite stable tumours (multivariate HR 1.85, 95% CI 1.15-2.99, P=0.012 and HR=1.52, 95% CI 1.01-2.29, P=0.044, respectively) and left-sided tumours (multivariate HR 2.66, 95% CI 1.45-4.86, P=0.002 and HR=1.76, 95% CI 1.03-3.02, P=0.039, respectively). CONCLUSIONS OCS classification stratified CRCs into three distinct subgroups with different clinico-molecular features and outcomes. Our findings provide a framework for a microbiota-based stratification of CRC to refine prognostication and to inform the development of microbiota-targeted interventions.