The research that changed how this conversation sounds came from post-mortem brain tissue. DNA from Porphyromonas gingivalis, the keystone bacterium in chronic gum disease, has been recovered from the brains of Alzheimer's patients at significantly higher rates than from cognitively normal controls. That is tissue analysis, not theory. How does mouth bacteria reach brain tissue. The current explanation is that P. gingivalis produces enzymes called gingipains that cross the blood-brain barrier. The bacterium also travels through the bloodstream and along the trigeminal nerve. An active periodontal infection gives it a direct route, refreshed every day, for years.