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
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 Negative 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
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.