How do extracellular pathogens cross the blood-brain barrier?

Bacterial invasion of the meninges can occur as a consequence of bloodstream invasion by some bacterial pathogens. Bacteria enter the central nervous system following a direct interaction with the luminal side of the cerebral endothelium, which constitutes the blood-brain barrier. To breach the barriers protecting the brain, extracellular pathogens must cross a monolayer of tight junction-expressing endothelial or epithelial cells. The limited number of pathogens capable of crossing these tight barriers and invading the meninges suggests that they display very specific attributes. For Neisseria meningitidis, type IV pili have been identified as being essential for meningeal invasion and it is believed other, as-yet-unidentified factors are also involved.