APP and Alzheimer disease: Transplantation of gut bacteria from AD patients to gut bacteria-depleted rats results in impaired neurogenesis and cognitive function.6 Gut bacteria-free Alzheimer’s precursor protein (APP)-transgenic or AppNL-G-F knock-in AD mice also exhibit reduced amyloid β peptide (Aβ) deposition and microgliosis in the brain.7-11 Consequently, gut bacteria play an important role in AD pathogenesis, though the mechanism of how gut bacteria impact brain pathology in AD remains to be investigated.