A 3-year-old boy from rural Maharashtra presents with a 2-day history of high fever (39.5°C), severe headache, neck stiffness, and photophobia. On examination, Kernig's and Brudzinski's signs are positive. CSF analysis shows: WBC 450/μL (85% neutrophils), protein 180 mg/dL, glucose 25 mg/dL (serum glucose 95 mg/dL), and Gram stain reveals pleomorphic Gram-negative coccobacilli. Blood and CSF cultures are pending. The child has not received Hib vaccination. What is the most likely causative organism?

Explanation

## Clinical Diagnosis: Haemophilus influenzae Type b Meningitis ### Key Microbiological Features **Key Point:** Pleomorphic Gram-negative coccobacilli on Gram stain is the pathognomonic morphology of Haemophilus influenzae. The organism appears as small (0.3–0.5 μm), pleomorphic rods or coccobacilli. ### CSF Profile Analysis | Parameter | H. influenzae | N. meningitidis | S. pneumoniae | L. monocytogenes | |-----------|---------------|-----------------|---------------|------------------| | **Cell type** | Neutrophilic (PMN) | Neutrophilic | Neutrophilic | Mixed/Lymphocytic | | **Protein** | Markedly ↑ (100–500) | Markedly ↑ (100–500) | Markedly ↑ (100–500) | Moderately ↑ (50–400) | | **Glucose** | Markedly ↓ (<40% serum) | Markedly ↓ (<40% serum) | Markedly ↓ (<40% serum) | Normal/↓ (>40% serum) | | **Gram stain** | Pleomorphic Gram-neg coccobacilli | Gram-neg diplococci | Gram-pos diplococci | Gram-pos rods | ### Clinical Context **High-Yield:** Haemophilus influenzae type b (Hib) is the leading cause of bacterial meningitis in unvaccinated children aged 2 months to 5 years worldwide. The absence of Hib vaccination in this rural Indian child is a critical risk factor. **Clinical Pearl:** The CSF glucose-to-serum glucose ratio of 25/95 (26%) is classically low in Hib meningitis, reflecting active bacterial consumption of glucose and impaired glucose transport across the inflamed meninges. ### Organism Identification 1. **Pleomorphic morphology** — Distinguishes H. influenzae from the diplococci of N. meningitidis and S. pneumoniae, and the rods of L. monocytogenes. 2. **Gram-negative** — Rules out S. pneumoniae (Gram-positive) and L. monocytogenes (Gram-positive). 3. **Age and vaccination status** — Unvaccinated 3-year-old is at peak risk for invasive Hib disease. 4. **CSF profile** — Neutrophilic pleocytosis with markedly elevated protein and low glucose is typical of Hib meningitis. ### Pathogenesis Hib is an encapsulated organism with a polyribose phosphate (PRP) capsule. The capsule is the primary virulence factor and is the target of the Hib conjugate vaccine. Unvaccinated children lack opsonizing antibodies and are susceptible to invasive disease. **Mnemonic: HACEK organisms** — H. influenzae, Aggregatibacter, Cardiobacterium, Eikenella, Kingella (fastidious Gram-negative organisms causing endocarditis; not relevant here but useful for differential diagnosis of Gram-negative organisms). ### Management Implications - **Empiric therapy:** Ceftriaxone or cefotaxime (3rd-generation cephalosporin) is first-line pending culture confirmation. - **Dexamethasone:** Adjunctive corticosteroid reduces neurological sequelae in Hib meningitis. - **Vaccination:** Post-recovery, ensure Hib vaccination to prevent recurrence. [cite:Park 26e Ch 19]