## 1 Introduction  
The gut microbiome influences both host metabolism and the immune system [1; 2; 3; 4; 5]. Some bacteria found in the gut are able to ferment indigestible dietary glycans into short-chain fatty acids (SCFAs) which play an important role in both gut health and overall health [6]. Acetate, propionate, and butyrate are the most predominant SCFAs involved in the maintenance of host health. Some important functions of SCFAs include that they are an energy source for intestinal epithelial cells, they help to strengthen the intestinal barrier, and they stimulate blood flow in the colon [7; 8]. Prior to stabilization, which occurs at around 3 years of age, the composition of the gut microbiome is dynamic and may be influenced by a number of factors. One such factor is antibiotic use, which is relatively widespread among children. The microbiota of antibiotic-treated children is modestly less diverse than untreated children, though this is only apparent in those >1 year of age [9]. Short-term microbiota composition changes are observed after antibiotic treatment in those <3 years of age [9; 10]. Decreased microbial diversity in early childhood may limit immune education, potentially leading to less robust microbial communities later in life [11].  
Probiotics are defined by the World Health Organization as \"live microorganisms which when administered in adequate amounts confer a health benefit to the host\" [12]. Several benefits are reported for probiotic use, including improved gastrointestinal health and gut barrier function, and beneficial effects on both the mucosal and systemic immune systems [13]. Members of the spore-forming _Bacillus_ spp. have been used as probiotics, having several advantages over non-spore-based probiotics. These advantages include resistance to desiccation and heat, and stability over a wide range of pHs, allowing them to survive the harsh acidic conditions of the stomach [14]. _Bacillus_-based probiotics have been shown to support increased SCFA production either directly or by promoting increased levels of SCFA-producing bacteria [15; 16]. An in vitro study demonstrated that these probiotics aid in the recovery of the adult gut microbiome after antibiotic-induced dysbiosis, particularly for propionate-producing bacterial families [15].  
The mucosal simulator of the human intestinal microbial ecosystem (M-SHIME(r)) is an in vitro modeling system that has been used to study the effects of probiotics, nutritional supplements, etc., on the activity and composition of the human gut microbiome [17; 18]. This model has several advantages, including integration of the entire gastrointestinal tract, the ability to maintain microbiome stability over a long period of time (up to 7 weeks), and that the microbiota of different age groups can be explored (e.g., adult, toddler, baby) [18; 19; 20].  
MegaSporeBiotic(tm) is a spore-based probiotic that contains five _Bacillus_ strains (_Bacillus indicicus_ [HU36], _Bacillus subtilis_ [HU58], _Bacillus coagulans_ [SC208], _Bacillus licheniformis_ [SL307], and _Bacillus clausii_ [SC109]). This probiotic supplement was previously shown to support changes in gut microbiome metabolism, including increased propionate production, in an M-SHIME(r) model of the healthy adult gut microbiome [21]. In that study, changes to the microbial community composition with MegaSporeBiotic(tm) supplementation were also observed. Given that the gut microbiome of children <3 years is still developing and that antibiotics are known to affect the gut microbiome of young children, with potential impacts on microbial communities later in life, this study aimed to evaluate whether MegaSporeBiotic(tm) had any effects on the recovery of the activity and composition of the gut microbial community of 6-24-month-old babies following treatment with a broad-spectrum antibiotic using the Baby-M-SHIME(r) technology platform.