BDNF and major depressive disorder: identified P. gingivalis and its gingipains in post-mortem Alzheimer’s disease (AD) brain tissue, providing direct pathogenic evidence linking oral infection to neurodegeneration (Dominy et al., 2019).Gut–brain axis: Duan and Wu summarized how gut microbiota modulate serotonin (5-HT), brain-derived neurotrophic factor (BDNF), and microglial activity via short-chain fatty acids and tryptophan metabolites, thereby influencing the pathogenesis of depression and Parkinson’s disease (Duan and Wu, 2024).