APP and early-onset autosomal dominant Alzheimer disease: Fecal Microbiota Transplantation (FMT) and Neuroinflammation: Fecal microbiota transplantation (FMT) in elderly APP/PS1 mice increased intestinal BACE1 and Aβ42 levels. Intestinal Aβ42 was transmitted to the brain through the bloodstream, activating microglia and enhancing neuroinflammation. This process contributes to the development of Alzheimer’s disease (AD).