## Distinguishing Feature: Capsule Presence **Key Point:** The polysaccharide capsule is the defining structural difference between encapsulated and non-typeable H. influenzae strains. Hib possesses a polyribose phosphate (PRP) capsule; NTHi lacks a functional capsule. ### Structural Comparison | Feature | Hib (Encapsulated) | NTHi (Non-typeable) | |---------|-------------------|---------------------| | Capsule | Present (PRP) | Absent or defective | | LPS | Present in both | Present in both | | Fimbriae | Present in both | Present in both | | X & V factors | Required in both | Required in both | | Virulence | High (invasive disease) | Moderate (mucosal) | ### Pathogenic Significance **High-Yield:** The capsule is the critical virulence factor that: 1. Resists opsonization and complement deposition 2. Evades phagocytosis 3. Enables invasive disease (meningitis, bacteremia, epiglottitis) 4. Is the target of the Hib vaccine (conjugate vaccine against PRP) **Clinical Pearl:** NTHi, despite lacking the capsule, causes significant disease through biofilm formation and mucosal invasion — particularly respiratory tract infections (otitis media, sinusitis, pneumonia, exacerbation of COPD). ### Why Capsule is the Best Discriminator While both strains share LPS, fimbriae, and growth factor requirements, only the capsule presence/absence directly correlates with: - Serotype classification (a, b, c, d, e, f for encapsulated; non-typeable for unencapsulated) - Invasive potential - Vaccine responsiveness - Clinical epidemiology (Hib → meningitis; NTHi → respiratory) **Mnemonic:** **PRP = Polysaccharide Ribitol Phosphate** — this is what makes Hib "H. influenzae type b."
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