BORN WITHOUT BUGS: EARLY MICROBIAL DEPLETION SHAPES BEHAVIOUR ACROSS ZEBRAFISH DEVELOPMENT

The gut microbiota is increasingly recognized as a critical modulator of brain function and behavior, influencing neuropsychiatric conditions such as anxiety and aggression. While previous studies have shown that germ-free (GF) zebrafish larvae display altered behavior and neurobiology, it remains unclear whether transient early-life microbial depletion induces lasting behavioral effects across development. This study aimed to assess the impact of early microbial depletion on zebrafish behavior across larval, juvenile, and adult stages. GF zebrafish larvae were generated and maintained under sterile conditions until 6 days post-fertilization (dpf), with GF status confirmed by 16S rDNA PCR. Experimental groups included GF larvae, conventionalized larvae recolonized with standard tank water (CV), and conventionally raised controls (CO). Larval behavior was assessed using a standardized light–dark transition assay in a Zantiks MWP unit. GF larvae exhibited significantly increased locomotor activity and reduced responsiveness to lighting transitions compared to CV and CO larvae. GF larvae also spent significantly more time in the inner zone during light phases, suggesting reduced anxiety-like behavior. Fish were subsequently raised to juvenile and adult stages, where anxiety-like behavior, aggression, and social behavior were assessed using standardized assays. At the juvenile stage, CV fish displayed increased locomotion, aggression, and social interaction. In contrast, adult GF fish exhibited increased anxiety-like behavior and a trend toward elevated aggression. These findings indicate that transient early-life microbial depletion and subsequent recolonization induce dynamic and stage-dependent behavioral alterations. Future studies will include neurobiological and microbiome profiling to identify underlying mechanisms.