Clinical meaning
Bacterial meningitis is a medical emergency in which pathogenic organisms (Streptococcus pneumoniae, Neisseria meningitidis, Listeria monocytogenes, Haemophilus influenzae) invade the subarachnoid space and multiply rapidly in the immunologically privileged cerebrospinal fluid. Bacterial cell wall components (lipopolysaccharide, peptidoglycan, teichoic acid) trigger a massive inflammatory cascade: resident macrophages and microglia release tumor necrosis factor-alpha and interleukin-1, which recruit neutrophils and increase blood-brain barrier (BBB) permeability, producing vasogenic edema. Antibiotic-induced bacterial lysis paradoxically releases a burst of cell wall fragments that amplifies this inflammatory response — this is the pathophysiological basis for adjunctive dexamethasone therapy. Dexamethasone, administered 15-20 minutes before or with the first antibiotic dose, suppresses the inflammatory cascade by inhibiting NF-kB, phospholipase A2, and pro-inflammatory cytokine production, reducing BBB disruption, cerebral edema, and neuronal damage. Clinical evidence demonstrates that dexamethasone reduces mortality in pneumococcal meningitis and hearing loss in H. influenzae meningitis, but provides no benefit if administered more than one hour after the first antibiotic dose (the inflammatory surge has already occurred). Empiric antibiotic selection is age-based: adults 18-50 years receive ceftriaxone plus vancomycin (covering penicillin-resistant pneumococcus); adults over 50 or immunocompromised patients require the addition of ampicillin for Listeria monocytogenes coverage (Listeria is intrinsically resistant to cephalosporins). Neonates receive ampicillin plus cefotaxime or gentamicin. The clinician integrates CSF analysis (cell count, protein, glucose, Gram stain, culture, multiplex PCR), blood cultures, and clinical severity to guide empiric therapy, determine dexamethasone appropriateness, identify indications for emergent imaging before lumbar puncture, narrow antibiotic coverage based on culture results, and manage complications including SIADH, seizures, cerebral edema, and hydrocephalus.