A child presents with invasive Haemophilus influenzae type b (Hib) meningitis. H. influenzae requires which growth factors for culture on blood agar?
- A Factor X (haemin) only
- B Both Factor X (haemin) and Factor V (NAD) ✓
- C Factor V (NAD) only
- D Neither — H. influenzae grows on plain blood agar without factors
Correct answer: B. Both Factor X (haemin) and Factor V (NAD)
Explanation
H. influenzae (non-typeable and typeable including Hib) requires both Factor X (haemin, heat-stable) and Factor V (NAD, heat-labile) for growth, which distinguishes it from other Haemophilus species. It shows satellite growth around S. aureus on blood agar because S. aureus lyses RBCs releasing haemin (X factor) and secretes NAD (V factor). Chocolate agar (heated blood agar releasing both factors) is the standard isolation medium. This X+V requirement is a classic identification feature.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.