The Preterm Infant Gut Microbiome and Probiotics Influence Growth Rates and Synaptic Development in Gnotobiotic Mice

Preterm birth increases the risk of neurodevelopmental disorders, potentially linked to impaired growth and neuroinflammation. Emerging evidence suggests that gut microbiome maturation, especially colonization by bifidobacteria, influences neurodevelopment via the gut-brain axis. However, mechanistic insights are limited by the lack of representative animal models. We investigated the impact of preterm infant-derived microbiomes on early growth and neurodevelopment by transferring fecal microbes (FMT) to germ-free C57Bl/6J mice. In one experiment, germ-free dams were colonized with fecal slurries from preterm infants with either high (>60%) or low (0%) bifidobacterial abundance (n = 5 per group). Pups were weighed on postnatal days (P)3, P5, P10, and P21. At P21, brains were collected for hippocampal immunofluorescence, and fecal microbiota sequencing was performed at both P10 and P21. In a second experiment, slurries from late preterm (+ high-bifido) or extreme preterm (+ low-bifido) infants were combined with maternal separation stress and FloraBABY probiotic treatment. Pups were weighed daily from P7 to P21; fecal microbiome sequencing was performed at P21. Across experiments, mice showed only partial colonization of donor microbiota, notably lacking bifidobacteria. In Experiment 1, high-bifido recipients exhibited reduced microbial richness at P21 (p < 0.05) and a shift in beta diversity (p < 0.001). In the context of neurodevelopment, they also had heavier brains and increased hippocampal Synapsin I (Syn1) expression (p < 0.001). In Experiment 2, mice receiving extreme preterm and probiotic treatment exhibited reduced weight gain (p < 0.05) and a shift in beta diversity (p < 0.001). Probiotic treatment also decreased alpha diversity (p < 0.05). Early-life microbiome composition was associated with prematurity-influenced growth and neurodevelopment in neonatal mice. Microbiome differences related to prematurity affected brain weight, synaptic markers, and body growth. However, the lack of bifidobacterial colonization in mice posed a key limitation of using mice to model the effects of human-specific microbiota. Probiotic treatment also unexpectedly reduced diversity and weight gain, underscoring the complexity and host-species dependence of early-life microbiome composition and responses to interventions.