The mode of action of probiotics may firstly be related to the modulation of the host's microbiota. One of the first suggested modes of action is the barrier effect,also called resistance to the colonization,exerted against pathogenic bacteria preventing or limiting their colonization. The bacterial inhibition may be due to the production of broad-spectrum inhibition bacteriocins,to metabolites such as short-chain fatty acids inducing a decrease of the pH little favorable for bacterial growth,or to biosurfactants with an antimicrobial activity.This barrier effect can also act via various mechanisms : competition for binding sits, inhibition of adhesion.