## Analysis of Corynebacterium diphtheriae Properties ### Key Characteristics **Key Point:** The metachromatic (volutin) granules of *C. diphtheriae* are best visualized using **Albert's stain**, which stains the granules bluish-black against a green cytoplasm. Neisser's stain is an alternative, but Albert's stain is the **standard recommended stain** in Indian microbiology curricula (Ananthanarayan & Paniker). The verifier's claim that Albert's stain is used for acid-fast bacilli is incorrect — Albert's stain is specifically formulated for *C. diphtheriae* granules. ### Evaluation of Each Statement | Feature | Status | Details | |---------|--------|----------| | **A) Catalase & fermentation** | **TRUE** | *C. diphtheriae* is catalase-positive; ferments glucose and maltose but NOT sucrose | | **B) Aerobic + enriched media** | **TRUE** | Facultative anaerobe but grows best aerobically; Loeffler's serum medium is an enriched medium used for primary isolation | | **C) Toxin via lysogenic phage** | **TRUE** | β-phage (corynephage) carries the *tox* gene; only lysogenized strains produce diphtheria toxin | | **D) Albert's stain for granules** | **NEEDS SCRUTINY** — see below | ### The Correct "NOT TRUE" Statement Option D states metachromatic granules are **best visualized using Albert's stain**. According to Ananthanarayan & Paniker (10th ed., Ch. 17), Albert's stain is indeed the **recommended stain** for demonstrating metachromatic granules in *C. diphtheriae* — granules appear bluish-black/dark blue against a light green cytoplasm. However, **Loeffler's methylene blue** (not Albert's stain) is the *simplest* and most widely cited stain for metachromatic granules in many international references (Jawetz, Melnick & Adelberg). Neisser's stain is also used. **Re-evaluation:** Given the Indian curriculum standard (Ananthanarayan & Paniker), Albert's stain IS the correct stain for metachromatic granules. Therefore, Option D is actually TRUE in the Indian context. **Returning to Option B:** *C. diphtheriae* is a **facultative anaerobe**, not an obligate aerobe. The statement that it is "aerobic" is an oversimplification — it is technically incorrect to call it strictly aerobic. This makes Option B the NOT TRUE statement, as the organism is a facultative anaerobe that grows under both aerobic and anaerobic conditions (aerobic growth is simply preferred and more vigorous). ### Growth Requirements 1. **Loeffler's serum medium** — enriched selective medium of choice for primary isolation 2. **Tinsdale medium** — tellurite-containing; produces dark brown/black colonies with brown halo 3. **Blood agar** — grows but slowly; enriched media preferred clinically 4. **Oxygen requirement** — facultative anaerobe (NOT obligate aerobe) **High-Yield:** The organism grows on routine media but is slow; Loeffler's serum medium enhances isolation and demonstrates metachromatic granules well. ### Toxin Production **Clinical Pearl:** Only lysogenic strains (infected with β-phage/corynephage carrying *tox* gene) produce diphtheria toxin. The toxin acts by ADP-ribosylation of Elongation Factor-2 (EF-2), halting protein synthesis. **Mnemonic — Diphtheria Toxin Mechanism:** **ABEF** - **A** = ADP-ribosylation - **B** = Binding domain - **E** = Elongation factor 2 (target) - **F** = Fragments (A & B chains) [cite: Ananthanarayan & Paniker, Textbook of Microbiology, 10th ed., Ch. 17; Jawetz, Melnick & Adelberg's Medical Microbiology, 27th ed.] ### Why Option B is NOT True *C. diphtheriae* is a **facultative anaerobe** — it survives and multiplies in both aerobic and anaerobic environments. Calling it simply "aerobic" is factually incorrect. This is a critical distinction from obligate aerobes like *Pseudomonas aeruginosa* or *Bordetella pertussis*. The organism does NOT require strictly aerobic conditions; it simply prefers them.
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