1 Microbiome diversity differs between AD elders and those with no dementia or other types of dementia. 2 Bacteroides, Alistipes, Odoribacter, and Barnesiella ↑; and Lachnoclostridium ↓ in AD elders. 3 Accurate classification of AD compared to those without dementia using metagenomic and clinical measures. 4 Accurate classification of AD versus other dementia types using only metagenomic measures. 5 AD microbiome induces lower P-gp expression. 6 Diminished butyrate enzyme-encoding genes in AD. Here, PGP is linked to Alzheimer disease.