Modulation of Toll-like receptor signaling by intestinal bacteria in colorectal cancer

The gut microbiota contributes to intestinal homeostasis through dynamic interactions with epithelial, immune and stromal cells. Disruption of the balance, often termed dysbiosis, has been associated with colorectal cancer (CRC). At the same time, selected commensal bacteria have shown protective effects. Toll-like receptors (TLRs) provide an important molecular interface through which host cells detect bacterial components and initiate cellular responses. This Mini Review examines current evidence that intestinal bacteria and their products modulate TLR signaling in CRC, with effects that may promote or restrain tumor development. The reported mechanisms involve CRC cells as well as immune and stromal compartments within the tumor microenvironment. The available evidence does not support assigning a fixed biological effect to a bacterial species or TLR. Instead, the reported outcomes appear to vary with the nature of the bacterial stimulus, the responding cell and the disease setting. Resolving this context dependence may clarify how host cells translate bacterial stimuli through TLRs into responses that influence CRC progression and treatment response.