Our gastrointestinal tract is colonized by trillions of microbes. There is growing recognition that commensal bacteria can promote the development of colorectal adenomas and cancer. For example, bacterial plaques are strongly associated with adenomas in familial adenomatous polyposis (Science 2018;359:592-597), and fusobacterium is associated with colorectal cancer liver metastasis.(Science 2017;358:1443-1448)