Bacterial Meningitis: Causative Organisms
The causative organisms of acute bacterial meningitis are clearly associated with the age of the patient. The common organisms in each age group are as detailed below:
Neonatal period: Escherichia coli, group B streptococci (S. agalactiae), and L. monocytogenes.
Ages 2 months to 5 years: H. influenzae
Above 5 years: S. pneumoniae and N. meningitidis.
In addition to the age of the patient, another factor that determines the causative pathogens is the presence of immunosuppression. In immunosuppressed patients, the most frequent pathogens include L. monocytogenes (especially in patients with cell-mediated immunity deficiency), S. pneumoniae (in patients with humoral immunity deficiency), and P. aeruginosa, enteric gram-negative bacilli, and S. aureus (in neutropenic patients).
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