TNF and Alzheimer disease: Recent data suggest a dual mechanism by which the oral microbiota and their metabolites contribute to brain dysfunction: directly, by crossing the blood–brain barrier and inducing local neuroinflammatory responses, and indirectly, by promoting systemic inflammation via chronic periodontal disease, which elevates circulating pro-inflammatory cytokines such as interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α), and interleukin-6 (IL-6), all implicated in the pathogenesis of Alzheimer’s disease [54,55,67,68,69,70,71,72,73].