Monocolonization with Bacteroides thetaiotaomicron exerts region-specific effects on Alzheimer's disease-related traits in the murine brain.

Bacteroides thetaiotaomicron (B. theta) dominates the gut microbiome of most mammals. This strictly anaerobic gut symbiont colonizes the mucus layer of host intestinal epithelial cells in both healthy and diseased conditions. Reduced neuronal and vagal afferent innervation observed in germ-free mice was found to be normalized by colonization with B. theta. In addition to deficits in gut innervation, germ-free mice have been reported to have reduced neuronal number and neurotransmitter levels in the brain. Here, we investigated the hallmarks of Alzheimer's disease (AD) in the brain of germ-free mice compared to mice mono-colonized with B. theta. We analyzed the number of mature neurons, neurotransmitter transporters, amyloid precursor protein processing, and inflammatory status in three brain regions: the hippocampus, prefrontal cortex (PFC), and cerebellum. The hippocampus and the PFC are regions thought to be highly susceptible to pathogenesis, whereas the cerebellum is thought to be only mildly affected. Interestingly, secretion of neuroprotective sAPPα decreased in hippocampus and remained unchanged in PFC, while levels were increased in the cerebellum in response to bacterial colonization. In addition, the number of presynaptic boutons increased in the hippocampus but remained unaffected in the cerebellum. IMPORTANCE: The gut microbiome has been reported to not only contribute to diseases of the gastrointestinal tract but also to interfere with and potentially even initiate diseases of other organ systems, such as the brain. Interference with the gut microbiome has been shown to elicit cognitive changes, for example, in rodent models of AD. Colonization with the common gut microbe B. theta not only affected the brain per se in our study but also showed specific brain region-dependent effects related to AD. This implies that evaluating the impact the microbiome might have on brain disorders needs a much more detailed investigation in the future with spatial and also potentially time resolution.