Bacteroides, a predominant commensal bacteria in the gut, are thought to be 
responsible for the development of inflammatory bowel disease (IBD). In the 
present study, we examined whether or not bifidobacteria suppress B. vulgatus, a 
representative pathogenic Bacteroides species, in both the coculture system and 
the gnotobiotic murine model. As a result, Bifidobacterium infantis 1222 highly 
inhibited the growth of B. vulgatus in the coculture and also significantly 
suppressed the systemic antibody response raised by B. vulgatus colonizing the 
gut in gnotobiotic mice. Colonization of the mice by B. vulgatus increased the 
number of Peyer's patch (PP) cells bearing PNA (peanut agglutinin)+/anti-kappa+ 
phenotype, which represents plasma cell-like B cells. Moreover, treatment of 
those B. vulgatus-implanted mice with B. infantis 1222 abrogated such increase 
in the number of PNA+/anti-kappa+ cells. These results thus suggested that B. 
infantis 1222 protected the gut epithelial layer including the PP from being 
invaded by Bacteroides, thereby suppressing the systemic antibody response 
raised by Bacteroides.