## 1 Introduction  
The use of bacteria to improve human health has been proposed for many years. The term probiotic has been defined by the Food and Agriculture Organization/World Health Organization (FAO/WHO, 2001) as \"live microorganisms which when administered in adequate amounts confer a health benefit to the host\" [1]. Probiotics have been shown to be useful in the oral bacteriotherapy of gastrointestinal and other disorders [2, 3]. The term of probiotic is mainly related to lactic acid bacteria, such as _Lactobacillus_ or Bifidobacterium_. However, it can be extended to other microorganisms that have not been fully explored, such as _Bacillus_ species, members of the allochthone microflora [4]. The use of _Bacillus_ strains as probiotics is supported by an increasing number of studies over the past 20 years, notably because of their ability to resist to the acid gastric environment under spore form [5; 6]. Moreover, _Bacillus_ spores are able to germinate into vegetative forms in the intestine if the conditions are favorable [7; 8; 9]. The consumption of _Bacillus_-based probiotics have also be shown to abolish the colonization by _S. aureus_ through the blockage of a pathogen's signaling system [10]. Although _Bacillus_ have been used as probiotics in different commercial preparations, such as LifeinUtm or Enterogermina(r), the mechanisms responsible for their beneficial effects remained relatively unexplored. _B. subtilis_ CU1 (LifeinUtm) product is a probiotic strain known to have an antagonistic activity against several pathogens, including _S. aureus_, _E. fecium_ and _C. difficile_[11]. Moreover, we previously highlighted the immunostimulatory properties of the lipoteichoic acid of the CU1 strain in vitro [12]. Interestingly, _B. subtilis_ CU1 can stimulate systemic and mucosal immune responses, increasing intestinal and salivary SlgA and serum IFN-gamma levels in humans [13]. In addition, strain CU1 displayed potent antidiarrheal activity in mice through the regulation of NHE3 and CFTR transporter proteins [14]. [PERSON] and [PERSON] [7] have shown that _B. subtilis_ could germinate in the small intestine, if nutritional germinants are available, leading to a brief colonization of the gut. For this reason, the addition of a prebiotic compound to the intake of probiotic spores could favor its germination once in the small intestine. By analogy, with probiotics, prebiotics were defined as \"nondigestible food ingredients that beneficially affect the host by stimulating growth and/or activity of a limited number of bacteria in the intestine\" [15; 16; 17]. To enhance the relative effect of each, combination of probiotics and prebiotics, named symbiotic, is a hopeful concept introduced by [PERSON] and [PERSON] [15]. Indeed, the prebiotic source could promote the survival and the establishment of a probiotic strain in the gastrointestinal tract [18]. Probiotics and prebiotics have been mostly investigated separately, but their respective advantages could work in a synergic manner and are yet to be explored. Among the prebiotic compounds commercially available, isomallocoligosaccharides (IMOSs) are produced by the enzymatic conversion of starch. IMOSs are composed of \(\alpha(1\to 6)\)- and \(\alpha(1\to 4)\)-linked glucose oligomers and mainly used as functionalized food in Asia. In vivo studies showed the effective role of IMOSs to increase bifidobacteria and lactobacilli in the gut of humans and rodents [19; 20; 21], and to decrease gas production in the gut [22]. However, the beneficial effects of the association of IMOSs with probiotic _Bacil_ have only been partially explored [23]. The aim of this study is to investigate the effects of a commercial prebiotic IMOS on the growth and metabolism of the probiotic _B. subtilis_ CU1, and to explore the possible role of this carbon source on the proteomic adaptation of the strain to gastrointestinal conditions. We first evaluate the growth abilities of _B. subtilis_ CU1 on several prebiotic sources, leading to the selection of IMOS as the prebiotic of interest.