## Most Common Cause of Bacterial Meningitis in Adults (18–50 years) **Key Point:** Streptococcus pneumoniae is the most common cause of community-acquired bacterial meningitis in adults worldwide, including India, accounting for 40–60% of cases in this age group. ### CSF Profile in Pneumococcal Meningitis | Feature | Finding | |---------|----------| | Cell count | 100–10,000/μL (typically 1,000–5,000) | | Cell type | Neutrophil predominance (>80%) | | Protein | Markedly elevated (100–500 mg/dL) | | Glucose | Low (<40 mg/dL or CSF:serum ratio <0.4) | | Gram stain | Gram-positive diplococci | | Culture | Positive in 70–90% of untreated cases | ### Epidemiology by Age Group **High-Yield:** The causative organisms vary by age: - **Neonates (0–3 months):** E. coli K1, Group B Streptococcus, Listeria - **Infants & children (3 months–5 years):** *S. pneumoniae*, *N. meningitidis*, *H. influenzae* type b (less common post-vaccination) - **Adults (18–50 years):** **S. pneumoniae** (most common), *N. meningitidis* - **Elderly (>50 years):** *S. pneumoniae*, *N. meningitidis*, *Listeria* ### Clinical Pearl The gram-positive diplococci morphology on Gram stain is pathognomonic for *S. pneumoniae*. The combination of low CSF glucose (due to bacterial consumption and impaired glucose transport) and elevated protein is classic for pneumococcal meningitis. **Mnemonic: PNEU** — *Pneumococcus, Neisseria, E. coli (neonates), Unforgettable (Listeria in elderly)* ### Risk Factors for Pneumococcal Meningitis - Asplenia or hyposplenism - CSF leaks (basilar skull fracture, cochlear implant) - Immunocompromise (HIV, malignancy) - Chronic alcoholism - Diabetes mellitus **Warning:** Do not confuse *Neisseria meningitidis* (gram-negative diplococci) with *S. pneumoniae* (gram-positive diplococci) on Gram stain — they have different epidemiologies and antibiotic sensitivities.
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