In the APP (Swe)/[PS1(L166P)] transgenic (Tg) model of AD, germ-free status reduced cerebral Aβ load compared to conventionally raised APP rodents (Harach et al., 2017), implicating microbiota as a direct or indirect modifier of peripheral amyloidosis (discussed later in Section “Gut Microbiome-Mediated Fibril Formation”). Here, PSEN1 is linked to Alzheimer disease.