Blood-borne immune cells carry low biomass DNA remnants of microbes in patients with colorectal cancer or inflammatory bowel disease.

The involvement of the intestinal microbiome in the pathogenesis of inflammatory bowel disease (IBD) and colorectal cancer (CRC), is well-established. Bacteria interact with immune cells at sites of intestinal inflammation, but also in the CRC tumor microenvironment. We hypothesized that bacterial remnants translocate within peripheral blood mononuclear cells (PBMCs) into the circulation and thus explored the composition of the detectable microbiome in PBMCs of patients with CRC or IBD compared to healthy controls. The PBMC microbiome profiles partially align with the tumor-derived or intestinal tissue-derived microbiome signatures obtained from the same patients with CRC or IBD, respectively. Our metagenomics data, supported by 16S-rRNA-FISH-Flow, imaging flow cytometry and species-specific qPCR, revealed the presence of translocated bacterial genetic sequences in the patients with CRC and IBD. Thus, our data suggest that in patients with intestinal barrier leakage, there is the potential for the translocation of bacterial remnants into the circulation via PBMCs.