Background Peptostreptococcus stomatis (P. stomatis) is an oral pathogen enriched in colorectal cancer (CRC) patients, but its causality and translational implication in CRC are unknown. In this study, we investigate the function of P. stomatis in CRC, and its impact on CRC therapy response.

Methods P. stomatis was identified as a CRC-enriched bacterium from multiple cohorts. The pro-tumorigenic effect of P. stomatis on colorectal carcinogenesis was determined in Apc^Min/+ mice and azoxymethane/Dextran sodium sulfate (AOM-DSS)-treated spontaneous CRC models. In vitro, function of P. stomatis was analysed by cell proliferation, apoptosis, and cell cycle assays. Interplay between P. stomatis and CRC cells was evaluated by fluorescence in situ hybridization (FISH), scanning electron microscopy (SEM), transmission electron microscopy (TEM), adhesion and invasion assays. The interaction between P. stomatis surface protein and the receptor of CRC cells was investigated by far-western, biotin pulldown, glutathione S-transferase (GST) pulldown, co-immunoprecipitation, and microscale thermophoresis (MST) assays.

Results P. stomatis accelerated colonic tumorigenesis in Apc^Min/+ and AOM-DSS models by inducing cell proliferation, suppressing apoptosis, and impairing gut barrier function. P. stomatis adhered to CRC cells through its surface protein fructose-1,6-bisphosphate aldolase (FBA) that binds to integrin α6/β4 receptor on CRC cells, leading to activation of ERBB2 and downstream MEK-ERK-p90 cascade. Blockade of FBA-integrin α6/β4 abolished ERBB2-MAPK activation and the protumorigenic effect of P. stomatis. P. stomatis-driven ERBB2 activation bypasses receptor tyrosine kinase (RTK) blockade by EGFR inhibitors (Cetuximab, Erlotinib), leading to drug resistance in xenograft and spontaneous CRC models of KRAS-wildtype CRC. P. stomatis also abrogated BRAF inhibitor (Vemurafenib) efficacy in BRAF^V600E-mutant CRC xenografts. E. coli overexpressing FBA phenocopied the effect of P. stomatis in promoting CRC. E. coli overexpressing FBA or recombinant FBA induced ERBB2-MEK-ERK-p90 signaling and promoted resistance to EGFR and BRAF inhibitors. Targeting ERBB2 reversed P. stomatis-associated resistance to EGFR and BRAF inhibitors in CRC cell lines and primary CRC organoids.

Conclusions We identified P. stomatis as an oncogenic bacterium and a contributory factor for non-responsiveness to RTK inhibitors in CRC.