JUVENILE GUT DYSBIOSIS INDUCES MICROGLIAL ACTIVATION, SELECTIVE POSTSYNAPTIC PRUNING, REGION-SPECIFIC METABOLIC SHIFTS, AND ANXIETY-LIKE BEHAVIOURS IN MICE

Early life represents a critical window for parallel maturation of the gut microbiome and central nervous system via the gut–brain axis. Antibiotic-induced dysbiosis during this period can destabilise microbial communities and relay inflammatory cues to the brain. This promotes persistent changes in synaptic remodelling and emotional processing, increasing vulnerability to neuropsychiatric disorders such as anxiety. Addressing key mechanistic gaps, this study investigated how early-life dysbiosis affects gut integrity, metabolic profiles, hippocampal microglial function, synaptic pruning, and anxiety-related behaviour in young mice. Male C57BL/6 mice received broad-spectrum antibiotics or vehicle via oral gavage from postnatal day 21–35, inducing robust gut dysbiosis confirmed by 16S rRNA sequencing, which revealed reduced alpha diversity and expansion of pathogenic genera, including Escherichia–Shigella. Dysbiotic mice exhibited caecal enlargement and altered defecation patterns, indicative of impaired gut homeostasis. Behaviourally, dysbiotic mice showed anxiety-like behaviour in the open field and elevated plus maze, while spatial memory (Y-maze) and sociability (three-chamber test) remained intact, suggesting limbic-specific vulnerability. Hippocampal CA1 immunohistochemistry revealed increased microglial activation with elevated IBA1 expression, reduced branching, and an amoeboid, pro-inflammatory morphology. This activation was accompanied by enhanced postsynaptic pruning, demonstrated by greater lysosomal engulfment of Homer1-positive postsynaptic terminals. Untargeted metabolomics further revealed distinct faecal metabolome profiles and region-specific metabolic perturbations in the prefrontal cortex, hippocampus, and cerebellum. Together, these findings establish a mechanistic cascade linking early-life gut dysbiosis to microglial-driven synaptic remodelling and anxiety phenotypes. They highlight novel microbiota–gut–brain axis pathways and support microbiome-targeted strategies for mitigating neuropsychiatric risk.