Previous studies have shown that germ-free APP/PS1 (amyloid precursor protein/presenilin 1) mice, have lower levels of Aβ, mainly Aβ42, in brain and plasma, whereas colonization by microbiota from aged APP/PS1 mice results in increased levels of Aβ, supporting a close cross-talk between gut and brain in AD [124]. This evidence concerns the gene APP and Alzheimer disease.