In that case the barrier is likely semipermeable. It may be that certain bacteria were advantageous to human evolution, so the barrier evolved with a mechanism that allowed good bacteria to pass through. Now this should make us humans a bit concerned about the likelihood of mimics. If good bacteria indeed play a role in human emotion and alertness, then perhaps some mimics result in certain types of mental illness.
I do not know a lot about bacterial meningitis, but I don't believe that it actually passes through the barrier. Meningitis is caused by the inflammation of a membrane that the brain and spinal chord are encased within [1]. So it might be causing the membrane to become enflamed from the outward facing surface.
There has been some potentially revolutionary work at CalTech with respect to the link between intestinal bacterial and Parkinson's (see link below). If it pans out it could potentially open up entirely new avenues of treatment.
Pesticides and plasticizers are known to disturb the blood-brain barrier and allow poisonous substances (and apparently bacteria?) to infiltrate brains.
