CNS Pathology

CNS Infection - Bacterial Identification

Laboratory testing algorithm for Neisseria meningitidis, Streptococcus pneumoniae (pneumococcus), Listeria monocytogenes, Hemophilus influenzae, E. coli, Group B Streptococcus, Staphylococcus aureus, and Pseudomonas aeruginosa

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Gram Negative Cocci - - Growth in High CO₂

Large Colonies on Chocolate Agar

Glucose Positive - - Maltose Positive

Neisseria meningitidis

Gram Positive Diplococci

Catalase Negative

Alpha hemolysis on blood agar

Optochin Sensitive

Streptococcus pneumoniae (pneumococcus)

Small Gram Positive Rods - Tumbling Motility

Growth: Wide Temperature Range

Catalase Positive

Small Pale Colonies - Beta Hemolysis

Listeria monocytogenes

Small Gram Negative Coccobacilli

Growth on Chocolate Agar

Hemin (X factor) and NAD (V factor

Required for Growth on Nutrient Agar

Haemophilus influenzae

Large Gram Negative Rods

MacConkey Agar - Positive Lactose

Positive Indole - - Green Sheen on EMB Agar

Escherichia coli

Gram Positive Cocci in Chains

Catalase Negative

CAMP Test Positive with

Beta Hemolysis on Blood Agar

Group B Streptococcus

(S. agalactiae)

Gram positive Cocci in Clusters

Catalase Positive - - Coagulase Positive

Staphylococcus aureus

Large Gram Negative Rods

Blood Agar Beta Hemolysis, Metallic Sheen

Oxidase Positive

Lactose Negative, Greenish Pyocyanin Pigment

Mueller-Hinton Agar Blue-green Pigment

Pseudomonas aeruginosa

CNS infectious organisms most often arrive via the bloodstream. The original portal of infection is often the respiratory tract. Alternatively the route of infection may be from sinuses, starting with otitis media, spreading to mastoid air cells, and then to the intracranial cavity. There can be inoculation via direct trauma when the skull is fractured.

The most common organism producing meningitis is a bacterium. Listed above are the most commonly encountered species. However, vaccines available for strains of Hemophilus influenzae, Streptococcus pneumoniae, and Neisseria meningitidis have altered the frequency with which such cases occur. The classic gross and microscopic findings are similar to exudative processes elsewhere caused by bacterial organisms, eliciting a predominantly acute neutrophilic inflammatory response.

The classic findings on examination of CSF include an increased number of WBCs (predominantly polymorphonuclear), low glucose, and elevated protein. The CSF opening pressure is elevated.

The list of organisms is the same across the life span, and the rank order can change by age and location.