MAPT and periodontal disorder: Our study indicates that an oral polymicrobial–periodontal infection/periodontal disease can extend the effects of periodontal pathogens, including P. gingivalis, T. forsythia and F. nucleatum from the oral cavity to the brain, and thereby induce a shift in the oral–brain–microbiome axis, produce neuroinflammation via production of proinflammatory cytokines, and produce Aβ and phosphorylated tau proteins in the brains of mice.