BackgroundOral microbial therapy has been studied as an intervention for a range of gastrointestinal disorders. Though research suggests that microbial exposure may affect the gastrointestinal system, motility, and host immunity in a pediatric population, data have been inconsistent, with most prior studies being in neither a randomized nor placebo-controlled setting. The aim of this randomized, placebo-controlled study was to evaluate the efficacy of a synbiotic on increasing weekly bowel movements (WBMs) in constipated children.MethodsSixty-four children (3–17 years of age) were randomized to receive a synbiotic (n= 33) comprising mixed-chain length oligosaccharides and nine microbial strains, or placebo (n= 31) for 84 days. Stool microbiota was analyzed on samples collected at baseline and completion. The primary outcome was a change from baseline of WBMs in the treatment group compared to placebo.ResultsTreatment increased (p< 0.05) the number of WBMs in children with low baseline WBMs, despite broadly distinctive baseline microbiome signatures. Sequencing revealed that low baseline microbial richness in the treatment group significantly anticipated improvements in constipation (p= 0.00074).ConclusionsThese findings suggest the potential for (i) multi-species-synbiotic interventions to improve digestive health in a pediatric population and (ii) bioinformatics-based methods to predict response to microbial interventions in children.ImpactSynbiotic microbial treatment improved the number of spontaneous weekly bowel movements in children compared to placebo.Intervention induced an increased abundance of bifidobacteria in children, compared to placebo.All administered probiotic species were enriched in the gut microbiome of the intervention group compared to placebo.Baseline microbial richness demonstrated potential as a predictive biomarker for response to intervention.