Abstract
BACKGROUND: Intratumoral pathogens are an emerging paradigm in metastatic colorectal cancer (CRC). Overgrowth of Enterococcus faecalis was shown to promote local recurrence in the colon, in a fashion dependent on collagenolytic virulence factors. The role of intratumoral enterococci in metastatic CRC is presently unknown. METHODS: We screened resected human metastatic CRC from the liver, lungs, and peritoneal surface for intratumoral bacteria with 16 s rRNA sequencing. We probed the effects of E. faecalis on CRC biology in vitro, with a focus on collagenolysis and the putative receptor for cleaved collagen, discoidin domain receptor 1 (DDR1) in CT26 CRC cells. We used a syngeneic, orthotopic mouse model of colorectal peritoneal metastases to measure the impact of E. faecalis on tumor bulk and immune infiltrate. RESULTS: Resected metastatic CRC from 70 patients were screened for intratumoral bacteria. Enterococcus species were identified in 10/13 patients with CRC peritoneal metastases and were enriched in peritoneal compared to non-peritoneal metastases. E. faecalis and CRC cells demonstrated cooperative collagenolysis in a fashion dependent on the secreted virulence factors GelE and SprE. Bacterial-induced collagenolysis led to increased DDR1 phosphorylation and downstream effects, specifically proliferation and endocytosis of cleaved collagen. In the mouse model, cell counts indicate intratumoral E. faecalis altered the immune compartment of the tumor microenvironment. DISCUSSION: Collagenolytic E. faecalis induce DDR1 pathway activation in CRC cells, alter the immune landscape in mouse models, and are enriched in human CRC peritoneal metastases. Further work is required to determine whether eradication of intratumoral bacteria can change tumor biology.