Increasing evidence suggests that changes in microbial composition in colon tissues are associated with progression of colorectal cancer (CRC). Commensal bacteria such as Fusobacterium nucleatum and Peptostreptococcus anaerobius, among others, are linked to CRC development via the modulation of the nuclear factor-kB immune cascade in the tumorous tissues. Immune surveillance, a process by which the immune system detects and destroys malignant cells, is key to control cancerous cells and tumour progression. Currently, the function of tissue-resident commensal bacteria and its interaction with the immune microenvironment for suppression of CRC development remains unknown. Here, Zhang et al. show that bacterial species belonging to the Lachnospiraceae family promote tumour immune surveillance activity of CD8+ T cells and protect against carcinogenesis.