578P Tissue-resident microbiota characterization in colorectal cancer metastases

Early evidence suggested a role of tissue-resident microbiota (TRM) in the development of colorectal cancer. However, its presence in colorectal cancer metastases (CRCM) is poorly reported. Here, we aim to explore and characterize the TRM in CRCM. From several biobanks in Belgium, we retrospectively collected 237 frozen samples, including 104 CRC primary tumor (PT) and 133 patient-matched CRCM (liver CRCM=98). Hepatocellular carcinoma (HCC, n=50), cholangiocarcinoma (CGC, n=23) and breast liver metastases (BLM, n = 16) were also collected. 82 CRCM were collected prospectively. The V4 region 16S rRNA gene was amplified and sequenced. We strictly applied sterile conditions from sample collection to sequencing, with the integration of negative controls (n=253). Additionally, 257 duplicated random samples were processed twice by independent experimenters. Sequencing results were processed with DADA2 to identify bacterial amplicon sequence variants (ASVs). ASVs were identified in all tissues. Microbial composition was significantly different between tumor samples and negative controls (PERMANOVA, p=0.0001). PT presented the highest bacterial biomass. ASV sharing frequency between PT and CRCM was significantly higher within than between patients (Wilcoxon test, p=0.007), suggesting potential bacterial translocation from PT to CRCM. The most prevalent PT bacteria were also the most shared in matched CRCM. No difference was observed between CRCM locations (liver, others). TRM of HCC, CGC and BLM were characterized by an ultra-low bacterial biomass, and distinct phyla compared to CRCM. In CRCM, the presence of Ruminoccocus gnavus (Rg) was associated with overall survival after CRCM resection (median 83.6 vs 41.6 months, HR: 0.35 IC95:0.13-0.90, p=0.024). We observe a specific TRM in CRCM, shared with matched PT and suggesting bacterial translocation from PT to CRCM. In this cohort, the presence of Rg in CRCM seems prognostic after CRCM resection. Further experiments are needed to confirm and detail these results.