Abstract: Quorum-sensing peptides are provided as diagnostic biomarkers, and quorum-sensing peptide inhibiting substances are provided for use in the treatment of metastasis of colorectal cancer in a subject, in particular a human subject. Methods for reducing metastasis of colorectal cancer in a subject include administering to the subject a microorganism that reduces or blocks activity of a pro-metastatic quorum-sensing peptide or a metabolite thereof present in a gastrointestinal tract or blood of the subject; and/or that reduces or blocks production of the pro-metastatic quorum-sensing peptide or the metabolite thereof in the gastrointestinal tract or blood of the subject. The pro-metastatic quorum-sensing peptide may include EntF or the metabolite EntF* of EntF.