A 6-year-old child from rural Maharashtra presents with acute epiglottitis. Blood culture grows a Gram-negative coccobacillus sensitive to ampicillin. Which feature distinguishes this organism from non-typeable H. influenzae (NTHI) that causes otitis media in the same age group?

## Typeable vs. Non-Typeable H. influenzae: Capsule as the Discriminator ### The Capsule Distinction **Key Point:** The presence or absence of a polysaccharide capsule is the defining feature that separates typeable H. influenzae (Hib) from non-typeable H. influenzae (NTHI). | Feature | Typeable H. influenzae (Hib) | Non-Typeable H. influenzae (NTHI) | |---------|------------------------------|-----------------------------------| | **Polysaccharide capsule** | **Present** (6 serotypes: a–f) | **Absent** | | **Serotype classification** | **Serotypeable** (a–f) | **Not serotypeable** | | **Virulence** | **High** (invasive disease) | **Lower** (mucosal disease) | | **Clinical presentation** | Epiglottitis, meningitis, bacteremia | Otitis media, sinusitis, pneumonia | | **Vaccine preventable** | **Yes** (Hib conjugate vaccine) | **No** | | **LPS endotoxin** | Present | Present | | **Fimbriae** | Present | Present | | **β-lactam sensitivity** | Variable (ampicillin-susceptible or resistant) | Variable | ### Clinical Pearl In this case, the organism causing epiglottitis in a child is almost certainly **serotype b (Hib)**, the most virulent and invasive form. The capsule confers antiphagocytic properties, enabling systemic dissemination and invasive disease. NTHI, lacking the capsule, is restricted to mucosal surfaces and causes localized infections (otitis media, sinusitis). ### High-Yield: Capsule = Virulence **Mnemonic:** **"Capsule = Catastrophe"** — typeable H. influenzae with its capsule causes severe invasive disease; NTHI without capsule causes mild mucosal disease. ### Why Capsule Matters 1. **Antiphagocytic property** — capsule masks PAMPs, evades innate immunity 2. **Serotype specificity** — capsule polysaccharide determines serotype (a–f) 3. **Vaccine target** — Hib conjugate vaccine (PRP-T, PRP-OMP) targets capsular polysaccharide 4. **Invasiveness** — capsule is essential for bloodstream survival and dissemination ### Pathogenesis Link The capsule enables Hib to: - Resist opsonophagocytosis - Cross the blood–brain barrier (meningitis) - Cause fulminant epiglottitis - Establish bacteremia NTHI, lacking this shield, remains localized to mucous membranes.