Microscopically, a gram stain reveals gram negative diplococci within a neutrophil, typical for Neisseria meningitidis. Gram stain and culture can be performed on cerebrospinal fluid obtained via lumbar puncture. A variety of bacteria can cause meningitis, but several are more common, and have an incidence more frequent at certain ages:
Escherichia coliNeonatesGram negative rods
Hemophilus influenzaeInfants and ChildrenGram negative coccobacilli
Neisseria meningitidisAdolescents and Young adultsGram negative diplococci
Streptococcus pneumoniaeOlder adults or ChildrenGram positive cocci in chains