APP and Alzheimer disease: Animal model studies have found that transplantation of feces from wild-type mice into AD mice (5XFAD, APP/PS1 mice) for 4 consecutive weeks can reduce Aβ deposition in the brains of AD mice, attenuate glial cell hyperactivation and the level of neuroinflammation secondary to it, and attenuate BBB permeability, thus improving the cognitive function of the mice (Sun et al., 2019; Kim et al., 2021).