Colonization of germ-free amyloid β precursor protein (APP) transgenic mice with gut microbiota from conventionally raised APP transgenic mice increased cerebral levels of the amyloid β-protein (Aβ), while colonization with the gut microbiota from wild-type mice was less effective in inducing these changes, indicating a role of the gut microbiota in AD development [178]. Here, APP is linked to Alzheimer disease.