Low power view of the inflammatory infiltrate in the subarachnoid space in a case of meninigtis. In acute meninigitis, the neutrophil is the most common cell, although as time goes on lymphocytes and macrophages can be seen.
Purulent meningitis is due most often to hematogenous spread of bacteria to the leptomeninges.
It can also be seen after head trauma as skull fracture through the sinuses. Local infections such as mastoiditis may also lead to meningitis. The most comman bacteria causing meningitis in neonates are streptococcus and E. Coli. In young children and adults Hemophilus influenzae, is common and in adults meningiococcus and pneumococcus are common. Bacteria extend through the wall of blood vessels into the subarachnoid space followed more slowly by neutrophils as the blood brain barrier breaks down.