Figure 1.

Direct and indirect mechanisms of infecting the brain. In the direct mechanism, the oral cavity infects the olfactory tract, and the olfactory nerve transfer the bacteria to the brain. In other mechanisms, bacteria inside the mouth infect the blood and find their way via blood, blood–brain barrier (BBB), perivascular spaces and circumventricular organs to the brain (figure is based on concepts presented in ref [51] and collected from ref [55])