A 5-year-old child with incomplete DPT vaccination develops acute respiratory distress, stridor, and a grayish pseudomembrane extending into the larynx. Blood culture and throat swab are sent. Which is the most common causative organism of this laryngeal diphtheria?

## Most Common Biotype of C. diphtheriae in Severe Diphtheria ### Biotype Classification and Virulence | Biotype | Frequency | Toxin Production | Colony Morphology | Clinical Severity | |---------|-----------|------------------|-------------------|-------------------| | **Gravis** | 40–50% | High | Large, flat, gray | **Most severe** (laryngeal, systemic) | | Intermedius | 30–40% | Moderate | Small, flat | Moderate | | Mitis | 10–20% | Variable | Small, mucoid | Mild-to-moderate | | Pseudo-tuberculosis | Rare | Minimal/none | — | Non-pathogenic | ### Key Point: **The gravis biotype of C. diphtheriae is the most common cause of severe diphtheria, including laryngeal involvement with airway obstruction.** It produces the highest levels of diphtheria toxin and is associated with systemic complications. ### High-Yield: The gravis biotype is identified by: - **Fermentation pattern:** Glucose and maltose fermented (with gas production) - **Colony morphology:** Large, flat, grayish colonies on Loeffler's medium - **Tellurite reduction:** Black colonies on tellurite medium (due to high lipid content) - **Toxin production:** Highest among all biotypes ### Clinical Pearl: Laryngeal diphtheria with stridor and airway obstruction is a medical emergency and is most commonly caused by the gravis biotype. This form carries the highest risk of myocarditis and neurological complications due to systemic toxin absorption. ### Mnemonic: **GIM** — Gravis (severe, gas-forming) > Intermedius (moderate) > Mitis (mild, mucoid) [cite:Textbook of Microbiology Baveja 6e Ch 22; Harrison 21e Ch 139]