The relationship between resident intestinal flora and colon cancer development are not yet clear. Apoptosis induction could represent a mechanism by which commensal and/or probiotic bacteria could prevent proliferation of dysplastic cells. In the present study, the in vivo and in vitro proapoptotic effect of resident bacteria was evaluated in mouse colon mucosa. Preliminary results suggest that colonic apoptosis induction, by commensal bacteria, could possibly represent a physiologic "oncologic surveillance" mechanism for colonic proliferative disease prevention.