Abstract

Background

Colorectal cancer (CRC) is the second leading cause of cancer related death in the UK, and is heavily influenced by environmental factors, including the gut microbiome. The development of CRC occurs over many decades, and some gut bacteria have been demonstrated to promote carcinogenesis. It is currently unclear whether bacteria play a prominent role in cancer initiation, or promote the progression of established tumours. The aim of this research was to compare the response of benign and malignant colorectal tumour cells to association with gut bacteria.

Method

Benign RG/C2 colorectal adenoma cells and HCT116 colorectal carcinoma cells were incubated with Enterotoxigenic Bacteriodes fragilis, Escherichia coli Nissle, Enterococcus faecalis and Fusobacterium nucleatum. Bacterial attachment and invasion was assessed using the gentamicin protection assay. The effect of invasion on cell yield was assessed, and the level of apoptosis determined by counting cells which had detached from the monolayer. Bacterial invasion and subcellular localisation was visualised using green fluorescent protein (GFP)-tagged bacteria and fluorescence microscopy.

Results

All species of bacteria were able to attach to and invade colorectal tumour cells in vitro. Similarly, all species induced proliferation in both RG/C2 and HCT116 cell lines (n=3, p<0.01). The induction of growth in HCT116 cells was consistent with previous literature, and the increase in cell yield of benign RG/C2 cells was comparable, with no significant difference in fold increase from control between the two cell lines. Levels of apoptosis in RG/C2 cells infected with E. coli Nissle or E. faecalis were significantly reduced (n=3, p<0.05), no reduction in apoptosis was seen in HCT116 cells.

Conclusion

These results show that constituents of the microbiome may influence pre-malignant tumours, potentially promoting tumour progression over time. Future work will focus on the ability of these bacteria to affect other aspects of tumour progression, including migration and invasion.