A 6-year-old girl presents with a thick, adherent, grayish-white pseudomembrane over the tonsils and pharynx. Gram stain shows gram-positive bacilli. Culture on Loeffler's serum medium shows small, grayish colonies. Which single feature best distinguishes a gravis biotype from an intermedius biotype of C. diphtheriae?

Explanation

## Biotypes of C. diphtheriae: Morphological Discrimination **Key Point:** C. diphtheriae is classified into three biotypes (gravis, mitis, intermedius) based on colony morphology on Loeffler's serum medium. Colony appearance is the primary discriminator. ### Biotype Comparison Table | Feature | Gravis | Mitis | Intermedius | | --- | --- | --- | --- | | **Colony size** | Large (1–2 mm) | Small (0.5–1 mm) | Small (0.5–1 mm) | | **Colony edge** | Irregular, jagged | Smooth, translucent | Smooth, translucent | | **Color** | Grayish-white | Grayish | Grayish | | **Texture** | Flat, dull | Shiny, glossy | Intermediate | | **Metachromatic granules** | Coarse, abundant | Fine, sparse | Fine, sparse | | **Fermentation pattern** | Maltose+ Glucose+ | Maltose− Glucose+ | Maltose+ Glucose+ | | **Toxin production** | Variable | Variable | Variable | | **Clinical severity** | Often severe | Often mild | Intermediate | **High-Yield:** Biotype classification is based on colony morphology on Loeffler's serum medium. Gravis produces large, flat colonies with irregular edges; mitis produces small, smooth, translucent colonies; intermedius is intermediate. Toxin production is NOT biotype-dependent — all three can be toxigenic or non-toxigenic. **Clinical Pearl:** Gravis biotype is classically associated with more severe systemic disease, but this is epidemiological, not biological. A non-toxigenic gravis strain causes mild local disease; a toxigenic mitis strain causes severe systemic disease. Biotype does not predict toxigenicity. **Mnemonic:** **GIM = Gravis (large Irregular Morphology), Intermedius (Intermediate size), Mitis (Minute, smooth)**