APP and Alzheimer disease: Comparing the Aβ plaque load between germ-free (GF) and specific pathogen-free (SPF) housing in the APP knock-in (AppNL-G-F) mouse model shows that the area and number of Aβ plaques are diminished in the hippocampus of GF housed AppNL-G-F AD mice compared to SPF housed AppNL-G-F AD mice (Figure 1(a,b)).