"Studies from Helsinki, Finland, have shown that kids fed a probiotic yogurt containing lactobacilli had a measurable reduction in new cavities. (Journal of Applied Microbiology, 2004.)
What appears to be happening is bacteria like to live in cozy communities called a biofilm. The growth of healthy biofilm communities is promoted by lactobacilli, commonly found in yogurt. Good bacteria, in sufficient quantities, muscle out the bad guys, leaving no place for them to attach their sticky fingers to your teeth."
Good bacteria can kick out cavity-causers | Walla Walla Union-Bulletin