Microbiome modulation and behavioural improvements in children with fragile X syndrome following probiotic intake: A pilot study

The gut microbiome (GM) is increasingly recognized as a key modulator of neurodevelopment via the microbiome-gut-brain axis. Fragile X syndrome (FXS), the most common inherited monogenic cause of intellectual disability, shares behavioural and molecular features with other neurodevelopmental disorders (NDDs), yet the role of the GM in FXS remains largely unexplored. In this open-label, single-arm trial, 15 children with genetically confirmed FXS received a daily probiotic formulation containing Lactobacillus casei, Lactobacillus salivarius, and Bifidobacterium breve for 12 weeks. Behavioural analysis and metagenomic sequencing with network and pathway analyses were performed before and after probiotic supplementation. Significant improvements were observed in irritability (-3.9, SD: ± 5.2; p = 0.027), communication (+ 1.7, SD: ± 2.5; p = 0.022), socialization (+ 1.4, SD: ± 2.1; p = 0.033), and adaptive behaviour (+ 1.3, SD: ± 1.4; p = 0.004). While overall microbial diversity remained stable, SparCC network analysis revealed increases in connectivity measures such as edge count and clustering coefficient, indicating denser microbial interactions and greater community coordination after probiotic supplementation. Functional profiling showed trends toward increased microbial activity in fatty acid biosynthesis, NAD salvage, and starch degradation pathways. This pilot study provides initial evidence that probiotics may modulate structural and functional properties of the GM, with potential links to improved behavioural outcomes in children with FXS. Larger, controlled trials are needed to validate the therapeutic potential of GM-targeted interventions in FXS and related NDDs.