Intratumoural Bacterial Co‐Occurrence is Associated With an Altered Tumour Immune Microenvironment and Poor Prognosis in Colorectal Cancer

ABSTRACT Alterations in the composition of the gut microbiota have been associated with colorectal cancer (CRC), with growing evidence indicating that specific microbes can have an impact on the tumour microenvironment. In this study, we investigated the associations of an oral pathobiont consortium comprising P. stomatis , F. nucleatum sensu lato and P. micra with the tumour immune microenvironment and the clinical presentation of CRC patients. Bacterial levels were quantified in fresh frozen tumour tissues using qPCR and the tumour immune environment was analysed using multiplex immunohistochemistry and multispectral imaging, profiling cytotoxic T cells, T‐helper 1 cells, regulatory T cells, B cells, macrophages and regulatory immune checkpoint molecules (CTLA‐4, PD‐1 and PD‐L1). Tumours with polymicrobial intratumoural co‐occurrence were more frequently right‐sided and exhibited microsatellite instability compared with tumours containing low bacterial levels or high levels of F. nucleatum sensu lato alone. Furthermore, polymicrobial tumours exhibited an altered tumour immune microenvironment, with decreased infiltration of FOXP3 + T regulatory cells and CD68 + macrophages, and increased tumour cell expression of PD‐L1 in the tumour centre. Polymicrobial tumours were also associated with poorer patient survival. In conclusion, our findings suggest a link between the co‐occurrence of P. stomatis , F. nucleatum sensu lato and P. micra and an altered tumour immune microenvironment in CRC. Elucidating the relationship between bacterial species and immune marker expression may enable the development of improved prognostic and predictive biomarkers, as well as new possibilities for therapeutic intervention.