This information comes from a March 2007 Journal of Nutrition article titled Probiotic Effects on Inflammatory Bowel Disease.
The beneficial effects of probiotics on IBD include:
(1) maintenance of intestinal homeostasis [maintaining a healty balance of gut flora]
(2) competitive exclusion of pathogens [keeping out harmful bacteria/pathogens]
(3) production of antimicrobial compounds
(4) promotion of the gut barrier function [stopping intestinal permeability/leaky gut]
(5) immune modulation ["crosstalk" involving intestinal bacteria regulates the inflammatory response]
More on this next week . . . the last few years of research are increasingly showing the critical role of intestinal bacteria in understanding/treating IBD.