Haemophilus influenzae requires which two growth factors for in vitro cultivation on standard media?

Explanation

## Haemophilus influenzae Growth Factor Requirements **Key Point:** Haemophilus influenzae is a fastidious gram-negative coccobacillus that requires two essential growth factors: **X factor (hemin)** and **V factor (NAD+)**, making it dependent on enriched media. ### X and V Factor Requirements | Factor | Chemical Identity | Source | Function | | --- | --- | --- | --- | | **X factor** | Hemin (iron protoporphyrin IX) | Blood | Cofactor for cytochrome oxidase and catalase | | **V factor** | Nicotinamide adenine dinucleotide (NAD+) | Blood | Cofactor for NAD+-dependent dehydrogenases | ### Media Selection for Culture **High-Yield:** Haemophilus influenzae grows on: - **Chocolate agar** (heated blood agar; X and V factors released by RBC lysis and heating) - **Levinthal medium** (enriched with X and V factors) - ~~Blood agar~~ (does NOT support growth — factors are sequestered in intact RBCs) - ~~MacConkey agar~~ (non-selective; lacks X and V factors) **Clinical Pearl:** The inability to grow on blood agar but growth on chocolate agar is a key **differential diagnostic feature** distinguishing Haemophilus influenzae from other respiratory gram-negative rods (e.g., Legionella, which grows on buffered charcoal yeast extract [BCYE] agar). ### Mnemonic for X and V Factors **"HeXamin and NAD"** — Hemin (X) and NAD (V) are the two essential factors. Alternatively: **"X marks the spot (hemin) and V is vital (NAD)"** ### Other Haemophilus Species and Factor Requirements | Species | X Factor | V Factor | Growth on Blood Agar | | --- | --- | --- | --- | | H. influenzae | Required | Required | No | | H. parainfluenzae | Not required | Required | No (V-dependent) | | H. haemolyticus | Required | Required | No | | H. ducreyi | Required | Not required | No (X-dependent) | **Warning:** Do not confuse X and V factors with other nutritional requirements. Some texts refer to "W factor" (thiamine) or "U factor," but these are not standard requirements for Haemophilus influenzae.