Abstract

Colorectal cancer (CRC) is a leading cause of cancer-related deaths globally. Incidence rates among individuals under 50 years are rising, which has led to the lowering of the recommended screening age from 50 to 45 years for those at an average risk. While numerous risk factors are associated with the development of CRC, the majority of cases contain microbial signatures representative of dysbiosis, indicating a role for the gut microbiome in disease pathogenesis. To date, most research has investigated microbiota independently of each other; however, it is widely accepted that microbiota interact with each other in the gut. More recently, two specific species of the microbiota have revealed a pro-carcinogenic synergism in vivo. Strains of both Bacteroides fragilis (B. fragilis) and Escherichia coli (E. coli) have been linked to CRC in clinical studies and been shown to induce carcinogenesis in mouse models through B. fragilis toxin (BFT) and colibactin, respectively. In this review, we discuss the roles of B. fragilis and E. coli in a healthy and diseased gut, current evidence associating each bacterium with CRC individually, and their synergistic contributions to the pathogenesis of CRC. Future investigation of CRC should focus on bacterial biofilms and additional potential pro-carcinogenic synergisms between other species of the gut microbiota to improve prevention and screening measures.