Significance Growing evidence indicates a correlation between colorectal cancer and intestinal dysbiosis or colonization by single bacterial species such as Streptococcus gallolyticus subsp. gallolyticus (SGG), yet a causality link remains to be established. To address this point experimentally, we colonized Apc ^{+/ −} Notch-inducible mice with SGG. Apc ^{+/ −} is a pioneer somatic mutation driving the occurrence of polyps. We did not observe significant changes in the occurrence or development of polyps in this model. However, we observed a strong SGG colonization in Apc ^{+/ −} mice at the expense of resident enterococci. We related this ecological substitution to activation of a SGG-specific bacteriocin whose activity is induced in vivo by the detergent effect of an increased concentration of secondary bile acids in relation to oncogenic situation.