Clinical meaning
Meningitis is inflammation of the meninges (pia mater and arachnoid) surrounding the brain and spinal cord. Bacterial meningitis is a medical emergency with mortality rates of 10-30% even with treatment. The most common causative organisms vary by age: neonates (Group B Streptococcus, E. coli, Listeria monocytogenes), children/adolescents (Neisseria meningitidis, Streptococcus pneumoniae), adults (S. pneumoniae most common, N. meningitidis), and elderly/immunocompromised (S. pneumoniae, Listeria, gram-negative bacilli). Pathogenesis involves nasopharyngeal colonization → bacteremia → crossing the blood-brain barrier (BBB). Bacteria multiply rapidly in CSF because the subarachnoid space lacks complement, immunoglobulins, and phagocytic cells. Bacterial cell wall components (lipopolysaccharide in gram-negatives, lipoteichoic acid in gram-positives) trigger intense inflammatory response: cytokine release (TNF-alpha, IL-1, IL-6), BBB disruption, neutrophilic infiltration, cerebral edema (vasogenic, cytotoxic, and interstitial), and elevated intracranial pressure. This inflammatory cascade, rather than direct bacterial toxicity, causes most of the neurological damage. Viral (aseptic) meningitis is more common and typically self-limited. Enteroviruses (coxsackievirus, echovirus) account for 85% of viral cases. HSV-2 causes recurrent lymphocytic meningitis (Mollaret meningitis). Fungal meningitis (Cryptococcus neoformans) occurs primarily in immunocompromised hosts (HIV with CD4 <100). Tuberculous meningitis...