TAC1 and major depressive disorder: On account of previous research, it can be concluded that chronic periodontitis is a contributor to certain neurological disorders including dementia, Alzheimer’s disease, and major depressive disorder through systemic mediators namely: IL-6, IL-1, TNF-α, CRP, alpha1-antichymotrypsin, IGF-2, neuropeptide Y, substance P, neurokinin A, calcitonin gene-related peptide, and vasoactive intestinal polypeptide, a pathogen consortium of porphyromonas gingivalis, tannerella forsythia, and treponema denticola bacteria.