Bacteroides fragilis modulates gut microbiome community composition, frontal cortex gene expression, and fecal and frontal cortex metabolites in a mouse model of Alzheimer’s disease pathologies

Alzheimer’s disease (AD) is a neurodegenerative disease and the leading cause of dementia among elderly. Gut microbiome alterations precede pathogenesis and may affect disease outcomes. We evaluated the role of Bacteroides fragilis in triple transgenic mice modeling AD pathologies (3xTg-AD) and wild-type controls (WT). Subsets of 3xTg-AD and WT mice were longitudinally treated with B. fragilis or sterile vehicle control for five consecutive days at 8 weeks of age and then monthly up to 52-56 weeks. Fecal samples were collected fortnightly from 8 through 52-56 weeks of age. Mice were sacrificed at 8 (baseline), 24 (amyloid-β plaques modeled), and 52-56 (amyloid-β plaques and neurofibrillary tangles modeled) weeks of age. Expression of genes involved in neuroinflammation and neurotransmission were quantified using reverse transcription quantitative polymerase chain reaction (RT-qPCR). Fecal bacterial microbiota were assessed by sequencing the V4 region of the 16S rRNA gene, and microbiome sequence data were analyzed using QIIME 2. Frontal cortex and fecal metabolomes were evaluated using LC-MS/MS. We observed that B. fragilis colonized the gut microbiota of 3xTg-AD mice earlier and more consistently than WT mice. 3xTg-AD mice treated with B. fragilis demonstrate lower gene expression of GFAP, SLC1A3, and FOXO3 in the frontal cortex. Consistent with this finding, treatment with B. fragilis restores levels of amino acid derivatives and neurotransmitters in 3xTg-AD mice to resemble levels in WT mice. These results highlight the role of the gut microbiome in AD-associated neuroinflammation and neurotransmission and the need for future studies to elucidate the mechanisms underlying these changes.