Bifidobacterium dentium Modulates Expression of Genes Important to Synaptic Plasticity and Neurodevelopment in the Cerebellum of Germ‐Free Mice
Authors: Luk Berkley , Engevik Melinda , Hall Anne , Versalovic James 
Published: November 2023

Background

Recent studies have established that members of the intestinal microbiota communicate with the host central nervous system (CNS) and suggest that this interaction may influence host neural development. Bifidobacterium species in particular are known to produce neuromodulatory metabolites, and previous studies have demonstrated that bifidobacteria can alter CNS gene expression, neurotransmitter function, and rodent behavior. In humans, bifidobacteria are detectable within the first week after birth and are the predominant genus in the infant intestinal microbiota. Given that Bifidobacterium spp. colonize the human system during a critical period of neural circuit development and organization, the Bifidobacterium‐gut‐brain interaction may have pervasive and lasting effects on brain function and behavior. However, we currently lack detailed knowledge regarding which CNS genes are altered by the presence of these species, and how modulation of these genes may affect neurodevelopment. We hypothesized that colonization with a representative Bifidobacterium species would alter CNS expression of genes important to synaptic plasticity and neurodevelopment.

Method & Results

Germ‐free mice were utilized to examine the effects of Bifidobacterium colonization. Adult male and female germ‐free mice were colonized with a representative species (B. dentium) by oral gavage. Age‐matched Specific Pathogen Free (SPF) mice that had a complex murine gut microbiome were also tested for comparison. We found that B. dentium colonization resulted in alterations to intestinal morphology, increased mucus production, and increased luminal concentrations of serotonin in the ileum. Due to its simple, well‐defined, and conserved circuit, the cerebellum was used as a model to study how the microbiota alters CNS gene expression and circuit development. Cerebellar gene expression was quantified via qRT‐PCR. Significant expression differences were observed in 10 of 30 synaptic plasticity‐related genes tested. Mono‐association with B. dentium resulted in a gene expression profile that resembled that of SPF mice for many of the genes tested, suggesting a rescue effect. Based on the qRT‐PCR results, 5 genes of interest were selected to examine via RNA In Situ Hybridization to further quantify and localize expression differences in whole brain sections. These results support the qRT‐PCR data, and have identified other brain regions of interest that exhibit altered gene expression. The marble bury test was utilized to assess variations in stereotypic digging behavior and indicated that B. dentium‐colonized mice displayed altered behavior relative to their SPF and germ‐free counterparts.

Conclusion

Together these data demonstrate that mono‐association of germ‐free mice with B. dentium alters intestinal morphology and neurochemical concentrations, as well as CNS gene expression which may correspond with the observed behavioral changes.