Pathogens most frequently involved in immunocompetent adults are Streptococcus pneumoniae (“pneumococcus,” ∼50%) and Neisseria meningitidis (“meningococcus,” ∼25%). Predisposing factors for pneumococcal meningitis include infection (pneumonia, otitis, sinusitis), asplenia, hypogammaglobulinemia, complement deficiency, alcoholism, diabetes, and head trauma with cerebrospinal fluid (CSF) leak. Listeria monocytogenes is a consideration in pregnant women, individuals >60 years, alcoholics, and immunocompromised individuals of all ages. Enteric gram-negative bacilli and group B Streptococcus are increasingly common causes of meningitis in individuals with chronic medical conditions. Staphylococcus aureus and coagulase-negative staphylococci are causes following neurosurgical procedures, especially shunting procedures for hydrocephalus.

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