In order to contribute to the comprehension of the mechanisms of actions of B. bifidum in the host’s intestine, here we set out to define in a mouse model the in vivo impact of the intake of B. bifidum on the microbiota and host’s gene expression in different intestinal sites. The same experiments were also performed with Lactobacillus helveticus MIMLh5, which has been chosen as a reference bacterium because this bacterium of dairy origin (Guglielmetti et al., 2008a) has been demonstrated to possess the ability to interact with the host (Guglielmetti et al., 2010; Taverniti et al., 2012; Taverniti et al., 2013). The present study demonstrates that, besides affecting the composition of the intestinal microbiota, the human isolate B. bifidum MIMBb23sg has a pronounced ability to affect host’s mucosal gene expressionof serotonergic and immunological mediators.. Bifidobacterium bifidum changes bacterial localization along the intestinal tract and modulates gut serotonergic metabolism

Bifidobacteria are strictly anaerobic, Gram-positive bacteria belonging to the Actinobacteria phylum, which have been largely employed as probiotics, i.e. “live microorganisms that, when administered in adequate amounts, confer a health benefit on the host” (Hill et al., 2014). Largely most of the microbial biomasses of probiotic bifidobacteria on the market consists of B. animalis subsp. Lactis, also due to its attitude to resist stresses during the industrial production; nonetheless, scientific literature demonstrates that also other Bifidobacterium species possess promising probiotic properties (Guglielmetti et al., 2011; Miraglia Del Giudice et al., 2017). Notably, peculiar health-promoting properties have been shown for those bifidobacteria that typically populate the intestinal microbiota of healthy breast-fed infants, such as B. breve, B. longum subsp. infantis and B. bifidum (Whorwell et al., 2006; Guglielmetti et al., 2011; Klemenak et al., 2015). In particular B. bifidum species can be considered an optimal example of human host-microbe co-evolution. This bacterium, in fact, possesses the ability to efficiently adhere to enterocytes (Guglielmetti et al., 2008b), utilize mucin as sole carbon source (Guglielmetti et al., 2009), catabolize human milk oligosaccharides (O'Callaghan and van Sinderen, 2016), and modulate host’s immune response (Guglielmetti et al., 2014a). Such features support the B. bifidum probiotic properties that have also been revealed during several clinical trials carried out for different human health conditions.