Saccharomyces boulardii is gaining in popularity as a treatment for a variety of diarrheal diseases, as well as inflammatory bowel disease. This study was designed to examine the effect of this yeast on infection by Shigella flexneri, a highly infectious and human host adapted enteric pathogen. We investigated key interactions between the bacteria and host cells in the presence of the yeast, in addition to a number of host responses including pro-inflammatory events and markers. While presence of the yeast during infection did not alter the number of bacteria that were able to attach or invade T-84 cells, it did positively impact the tight junction protein, Z0-2, and significantly increase barrier integrity of model epithelia. The yeast also decreased ERK, JNK and NF-?B activation in response to S. flexneri, events likely responsible for the observed reductions in IL-8 secretion and the transepithelial migration of polymorphonuclear leukocytes (PMNs) across T-84 monolayers. These results, suggesting the yeast allowed for a dampened inflammatory response were confirmed in vivo utilizing a highly relevant model of human fetal colonic tissue transplanted into scid mice. Furthermore, a cell-free S. boulardii culture supernatant was also capable of reducing IL-8 secretion by infected T-84 cells. These data suggest that while the use of S. boulardii during infection with S. flexneri may alleviate symptoms associated with the host's inflammatory response, it would not prevent infection.