Regarding the virulence factors and laboratory identification of Corynebacterium diphtheriae, all of the following are correct EXCEPT:

Explanation

## Virulence Factors and Identification of C. diphtheriae ### Evaluation of Statements | Statement | Status | Explanation | |-----------|--------|-------------| | Tinsdale medium (dark halo) | **INCORRECT** | The dark halo around colonies on Tinsdale medium is due to H₂S production by C. diphtheriae reacting with potassium tellurite — NOT simple reduction of tellurite. The black colony color itself results from tellurite reduction, but the characteristic **halo** is due to H₂S reacting with tellurite salts in the medium. | | Toxin mechanism (EF-2 ADP-ribosylation) | **CORRECT** | Diphtheria toxin catalyzes ADP-ribosylation of EF-2 (elongation factor 2), halting protein synthesis — a classic high-yield fact | | Hyaluronidase & capsule | **CORRECT** | C. diphtheriae does NOT produce a hyaluronic acid capsule; this statement is false, making it a plausible distractor, but it is NOT the best "except" answer here | | Schick test | **CORRECT** | Intradermal injection of diphtheria toxin; induration indicates susceptibility (no antitoxin) | ### The Critical Error: Tinsdale Medium Mechanism **Key Point:** On Tinsdale medium, C. diphtheriae produces **two distinct features**: 1. **Black colonies** — due to reduction of potassium tellurite to metallic tellurium (Te⁰) 2. **Dark brown/black halo** around colonies — due to **hydrogen sulfide (H₂S) production** by the organism, which reacts with potassium tellurite in the medium to form a brown sulfide precipitate Option A incorrectly states that the dark halo is produced "due to reduction of potassium tellurite." This is inaccurate — the halo is specifically due to H₂S production, not tellurite reduction. Tellurite reduction accounts for the black colony color, not the halo. This distinction is well-established in standard microbiology references (Ananthanarayan & Paniker, Mackie & McCartney). **High-Yield:** The halo on Tinsdale medium = H₂S + tellurite reaction. The black colony = tellurite reduction. These are two separate phenomena. ### Virulence Factors of C. diphtheriae 1. **Diphtheria toxin** (phage-encoded, tox gene from β-phage) - Fragment A: ADP-ribosylates EF-2 → inhibits protein synthesis - Fragment B: Binds host cell receptor (HB-EGF precursor) 2. **Adhesins** (pili/fimbriae) — attachment to respiratory epithelium 3. **Iron-binding proteins** — compete for iron in host tissues 4. **NO true capsule** — C. diphtheriae is unencapsulated ### Laboratory Identification **Mnemonic — C. diphtheriae ID:** **LATTE** - **L** = Loeffler's serum medium (primary isolation, metachromatic granules) - **A** = Albert's stain (demonstrates metachromatic/volutin granules) - **T** = Tinsdale medium (black colonies + brown halo) - **T** = Toxin production testing - **E** = Elek test (immunodiffusion for toxin confirmation) ### Schick Test - **Intradermal injection** of diphtheria toxin (0.1 μg) - **Positive (induration)** = susceptible (no protective antitoxin) - **Negative** = immune (has antitoxin) - **Pseudo-reaction** = local irritation only (non-specific) **Clinical Pearl:** The Schick test is now rarely used in vaccinated populations but remains a classic historical marker of immunity assessment and a high-yield exam topic. [cite: Ananthanarayan & Paniker Textbook of Microbiology 10e Ch 17; Mackie & McCartney Practical Medical Microbiology 14e; Jawetz, Melnick & Adelberg's Medical Microbiology 27e] ### Why Option A Is the EXCEPT Answer Option A states the dark halo is due to "reduction of potassium tellurite" — this is factually imprecise. The halo is produced by H₂S reacting with tellurite, not by direct tellurite reduction. Tellurite reduction produces the black colony color. This makes Option A the incorrect statement among the choices.