Slide 1. Streptococcus pneumoniae meningitis
This cerebrospinal fluid from a child with meningitis contains many neutrophils and oval gram-positive cocci singly and in pairs. Because the number of organisms in infected cerebrospinal fluid is small, most laboratories centrifuge the specimen to increase the concentration and then use the sediment for both stains and cultures. The density of
microbes per milliliter of fluid cannot be estimated from a specimen that has been centrifuged.

Slide 4. Listeria monocytogenes meningitis
This cerebrospinal fluid contains a few neutrophils and two slender gram-positive bacilli. Although Gram stains of cerebrospinal fluid are positive in specimens from about 80% of all patients with bacterial meningitis, organisms are detected in the cerebrospinal fluid of only about 40% of patients with Listeria meningitis. Even when specimens reveal bacteria, only a small number may be visible.