A 3-year-old unvaccinated child presents with acute onset of fever, neck stiffness, and altered sensorium. CSF analysis shows turbid fluid with elevated protein (120 mg/dL), low glucose (15 mg/dL), and pleocytosis with predominant neutrophils. Gram stain reveals gram-negative coccobacilli. What is the most common causative organism of bacterial meningitis in this age group?

## Epidemiology of Bacterial Meningitis by Age **Key Point:** Haemophilus influenzae type b (Hib) was historically the most common cause of bacterial meningitis in children aged 2 months to 5 years before the introduction of Hib conjugate vaccine. In unvaccinated populations, it remains the most frequent cause in this age group. ### Clinical Presentation The CSF findings described—turbid appearance, elevated protein, low glucose (CSF:serum glucose ratio <0.4), and neutrophilic pleocytosis—are classic for Hib meningitis. The gram-negative coccobacilli morphology on Gram stain is pathognomonic for Haemophilus species. ### Age-Related Epidemiology | Age Group | Most Common Organism | Second Most Common | |-----------|----------------------|--------------------| | 2 months–5 years (unvaccinated) | Haemophilus influenzae type b | Neisseria meningitidis | | 2 months–5 years (vaccinated) | Streptococcus pneumoniae | Neisseria meningitidis | | >5 years | Neisseria meningitidis | Streptococcus pneumoniae | | Neonates (0–28 days) | Group B Streptococcus | Escherichia coli K1 | **High-Yield:** In vaccinated populations, S. pneumoniae has become the leading cause in young children, but in unvaccinated or partially vaccinated children (common in resource-limited settings), Hib remains the most frequent pathogen. ### Virulence Factors of Hib **Clinical Pearl:** The type b polysaccharide capsule is the major virulence factor. Unencapsulated strains and non-b serotypes cause disease much less frequently. The capsule resists opsonization and complement-mediated lysis, allowing hematogenous dissemination to the meninges. **Mnemonic:** CHAMP — Common causes of meningitis in Children: **H**aemophilus influenzae (unvaccinated), **C**occus (Meningococcus/Pneumococcus), **A**ge-dependent, **M**eningitis, **P**revention by vaccine. ### Why Hib Causes Meningitis 1. Nasopharyngeal colonization 2. Invasion of respiratory epithelium 3. Bacteremia 4. Blood–brain barrier penetration 5. Meningeal inflammation and CSF infection **Warning:** Do not confuse serotypes. Type b is the most virulent and vaccine-preventable. Non-b serotypes (a, c, d, e, f) and unencapsulated strains rarely cause invasive disease.