Which feature best distinguishes Haemophilus influenzae from Haemophilus parainfluenzae?

Explanation

## Distinguishing H. influenzae from H. parainfluenzae ### Growth Factor Requirements **Key Point:** The cardinal difference between these two Haemophilus species lies in their nutritional requirements for growth on culture media. | Feature | H. influenzae | H. parainfluenzae | |---------|---------------|-------------------| | **X factor (hemin) requirement** | **Required** | Not required | | **V factor (NAD) requirement** | **Required** | Required | | **Growth on blood agar** | Around Staph colonies (satellite growth) | Grows independently | | **Growth on chocolate agar** | Grows well | Grows well | | **Fermentation pattern** | Glucose, sucrose, lactose | Glucose only | | **β-lactamase production** | Variable (30–50% of strains) | Variable | **High-Yield:** H. influenzae's absolute dependence on **both X and V factors** is the gold-standard discriminator. This is why it shows characteristic satellite growth around Staphylococcus aureus colonies on blood agar (Staph produces V factor; hemin is present in blood). H. parainfluenzae, lacking the X factor requirement, grows independently. ### Clinical Pearl In the laboratory, if a Gram-negative coccobacillus grows on chocolate agar but NOT on plain blood agar (unless near a Staph colony), suspect H. influenzae. Conversely, growth on blood agar alone suggests H. parainfluenzae. ### Why Factor Requirements Matter 1. **X factor (hemin)** — essential for cytochrome synthesis in H. influenzae 2. **V factor (NAD)** — required for energy metabolism in both species 3. H. parainfluenzae can synthesize its own hemin, eliminating the X factor dependency **Mnemonic:** **"X marks the spot for H. influenzae"** — only H. influenzae needs X factor; this is the distinguishing feature.