Fig. 3: Microbes of the oral cavity and stomach communicate through the upper gastrointestinal tract.

The representative oral microbiota in healthy individuals contains the major gastric microbiota in healthy individuals, although most oral microbes are blocked by the mucus barrier secreted by the healthy gastric mucosa. However, under certain circumstances, such as persistent H. pylori infection, the weakened gastric mucus barrier allows oral-associated microbes to invade. Ectopically colonized microbes promote gastric carcinogenesis through direct contact or products. Oral and gastric dysbiosis have potential effects on the brain, probably triggering feedback on lifestyle and self-maintenance. The direct effects and indirect effects are shown by solid and dashed arrows, respectively. Created with BioRender.com.